CCleaner was originally known as Crap Cleaner and cleaned up its name when it went commercial.
As far as I've ever known, file cleaning is not done to improve "speed" of the computer but to reclaim what would otherwise be dead disk space. I've always found it pretty appalling how many programs leave all kinds of garbage lying around from installation, uninstallation, and otherwise.
Registry cleaning is what I've always heard claims made regarding improving overall speed, and whether it does or not at least the theoretical basis for that claim has surface validity. I don't do it anymore, though, as it doesn't seem to have any effect that I've ever been able to detect.
Cleaning browser cache and optimizing the databases they use definitely has a positive impact on performance and often solves actual problems with their behaviors.
Brian AKA Bri the Tech Guy (my website address is in my profile) Windows 10 Home, 64-bit, Version 1709, Build 16299
Here is a test to find out whether your mission in life is complete. If you’re alive, it isn’t.
~ Lauren Bacall