I avoid these 5 PC optimization tools that just don’t help

It's a lot easier to take care of a PC than it used to be. I'm old enough to remember DOS, and often, just getting PC games to run was a battle -- if it wasn't a problem with the IRQ settings on a sound card, it was having to load something like EMM386 just to have enough memory. Things are much, much easier today, and I'm here for it.
People continue to use tools to tweak Windows and Mac machines, but mostly to get optimum performance, or prevent issues from cropping up. Some of these can be invaluable. Others, though, can be redundant, or actively harmful to your computer, whether because they're outdated or because they're outright scams. This quick primer should help you steer clear of the apps you shouldn't bother with.
Drive defragmenters
Increasingly irrelevant, likely dangerous
Back when platter (HDD) drives were common, defragmenting them was a necessity. As an HDD writes and rewrites, bits of data can get scattered across the disk, gradually increasing seek time for the reader. Defragmenting undoes this decay, allowing the platter to spin as efficiently as possible.
Here's the thing -- outside of servers and network-attached storage (NAS) systems, few people use HDDs anymore. Most computers rely on SSDs (solid state drives), which are based entirely on flash storage, i.e. memory chips. Rearranging sectors isn't going to provide a meaningful speed improvement, since there are no moving parts.
Flash chips have a finite number of write cycles, and defragmenting them will aggressively use those up, shortening your SSD's lifespan.
In fact, trying to defragment an SSD is one of those things that's actively harmful. Flash chips have a finite number of write cycles, and defragmenting them will aggressively use those up, shortening your SSD's lifespan. You should never, ever run a defrag on an SSD.
SSDs do need to undergo a process known as trimming, which removes "garbage" data and flags that space as available for writing again. This should happen by default if you're using Windows 10, 11, or any recent version of macOS, so there's no need to install any third-party TRIM tools.
Windows Registry cleaners
A minor improvement at best
The Registry is a vital component of Windows, serving as a database for low-level settings. Over time, actions like uninstalling apps can leave empty, redundant, or broken entries, slowing this database down or creating conflicts. In some scenarios, they can prevent apps or Windows from booting properly.
You'd think, then, that a registry cleaner might be essential, but Microsoft's own software has improved enough to make this irrelevant, all the more so when it comes to speed. Your current PC is probably capable of 4K video, and with the right graphics tech, photorealistic 3D -- removing a few empty registry entries isn't going to provide a noticeable speed boost. One analogy I saw recently likened it to removing a couple of blank pages from a 10,000-page dictionary.
Worse, a registry cleaner can sometimes flag Registry items by mistake. If that happens, the cleaner itself may sabotage Windows, causing more problems than it solves. Troubleshooting the Registry is sometimes a necessity -- but it's best to rely on Microsoft's own tools or manual, well-educated edits.
Third-party antivirus suites
Another relic of a bygone age
By now, the recurring theme should be obvious. There was a time when third-party antivirus (a.k.a. anti-malware) tools were vital, since computers didn't ship with them out of the box. This is despite viruses predating the explosion of the internet in the late 1990s -- there was chance that copy of Doom your friend pirated for you could be infected.
Today, both Windows and macOS feature robust antivirus tools with real-time protection and frequent updates. They're not bulletproof, but no piece of software can be. The only way to fully protect yourself from viruses is to airgap a machine, and few people can afford to go completely offline.
There are legitimately useful antivirus suites out there, such as Malwarebytes, yet they're likely redundant if your operating system's built-in tools are running and you're checking for updates on a frequent basis. Perhaps the best reason to avoid third-party tools is cost -- while they may start out free, there's usually a hard push to pay for a "premium" version, and some developers won't leave you alone until you subscribe.
Save yourself some cash. Speaking from experience, I've found that Windows Security, two-factor authentication, and my web browser's anti-phishing protections have been sufficient to keep me safe. Skeptical thinking does wonders, too.
Aggressive CPU and GPU optimization utilities
Tread carefully
The giant asterisk with this one is that if an app comes from your PC or GPU maker, there shouldn't be anything inherently dangerous about it. Indeed you probably should take advantage of this software, since first-party tools are designed to eke out the most performance possible without pushing specs into the danger zone. Your computer defaults to more conservative settings as a precaution.
Where things get complicated is with third-party utilities. These may be fine in the right hands, but they're often geared towards hobbyists who understand their system's limits and are willing to take risks. If you overclock a component too far, it could make your PC unstable or shorten its lifespan. Even undervolting can potentially lead to crashes and freezes, no matter that it's intended to make a PC quieter and more power-efficient.
I find that first-party apps from companies like Razer, Lenovo, and Nvidia are more than good enough for my purposes. I should caution that you have to pay attention to how they interact, just in case there are cumulative effects. Also, even on their own, the risks aren't nil. If a GPU is operating faster than usual, it's automatically running hotter, which could cause trouble if your PC doesn't have adequate internal cooling or room ventilation.
Disk cleaners
It's just not that hard, people
It's important to clean up space on your hard drives from time to time. Bloated system caches can impact performance, or lead to bugs if files become corrupt. You also need a base amount of room for other temporary files -- if you fill a drive to the brim, some functions may run slowly, if at all. Mainly I'm thinking of software updates, but you could also encounter barriers with games, video editing, and audio production.
Windows, macOS, and some individual apps have their own options for managing temporary files, and if you need more room, it's best to do things the hard way.
There's really no need to download a third-party cleaning utility, though. Windows, macOS, and some individual apps have their own options for managing temporary files, and if you need more room, it's best to do things the hard way: by uninstalling apps you're no longer using, or hunting for junk documents and media. You can recover dozens or hundreds of gigabytes by removing games you're no longer playing, or movies and shows you've lost interest in.
Disk cleaners can automate this process, but they're capitalizing on ignorance, laziness, or both. Some may actually be scams. Either way, your time and money should go elsewhere.