PC Cleaner

Can but better not. One main points of the imaging/cloning is to make sure your backup system image is safe. Including it in the same harddrive sounds like inviting disaster if the said HDD is failing.

I've used either, and found both are suitable for my needs.

I'm using FREE versions, though.

Windows reads the registry for almost everything you do. The registry tells windows where to find and how to use the vast majority of the programs and services that run, including all your background services that are always on.

The more bloated your registry becomes, with old unused registry keys via uninstalled programs that didn't clean their registry entries out, various updates adding new entries and invalidating old, and so on, the longer the time to read through the registry is.

A fair amount of the complaints people have of their computers "feeling slower" over time can be traced back to file system bloat, and registry bloat.

Programs like CCleaner (which is the only one I will use personally) will go through and remove the vast majority of them, and genuinely do help your computer start many programs faster, sometimes a very noticeable amount. I generally try to run CCleaner about once every 3 months or so - more often if I've gone on a binge of downloading Steam games and such from bundles to try and later uninstalled.

I COULD go do this manually, however it's very annoying and time consuming, so it's far easier (lazier) to use a program like CCleaner to do it for me.

As you mentioned, it's not technically without risk of "harming your software" in that it could, in theory, mistakenly remove an entry for a program that is still installed and create some issues. That said, I've been using CCleaner for like 5 or 6 years now I think, and I have yet to ever encounter that. Meanwhile I recently manually removed a single program's clusterfuck of registry files from a friends computer for him because the uninstall didn't do so and so the old registry files were preventing him from installing an updated version, and doing it manually took the better part of 30 minutes.

Extrapolating that out to a more reasonable estimate per program of about half that time, and assuming at least 10 programs worth of time every three months or so, and in the 5 years I've used that program would have saved me somewhere around 37.5 hours I would otherwise have spent manually cleaning stuff, or alternatively having had more slowdown/error issues to fix, or more frequent Windows re-installations to deal with. The reality is that the time I've saved is probably considerably higher than that. Personally I would call the risk-reward ratio of that hugely in my favor, since if it ever did erroneously damage one program, fixing that would only take like 30 minutes or less. Risking 30 minutes of work to save 38 HOURS sounds totally worth it to me.