The thing is, the need for system optimization tools came about years back with the legacy windows (Win 95, 98, ME etc..) because the operating systems and third party programs were not very efficient, not well made or limited in functionality. For example if you uninstalled something it would always leave registry value's, broken links an all kinds.
But it was the fact that hardware wasn't as advanced back then as it is these days, the biggest problem of PC performance is the Hard Drive [HDD] and the specific bus system it uses (IDE, SCSI, Sata, Optical etc...), because it's reading and writing so much data every second that if you had a lot of files scattered (fragmented) all over the HDD it would clearly need more time to read them all, hence the need for disk defragmentation, this
was technically "System Optimization".
But these days you have so many high transfer connectivity available, not forgetting high rpm HDD's and
even solid state (not forgetting the advancement in all other components), the need to 'optimize' is low; and basically they
"clean" more than
"optimize".
Most optimizers do other bits and bobs but mainly:
1. Delete random files not associated with any program
2. Clear most or all internet cache and/or temporary folders
3. Delete index.dat files that misteriosly get bigger but still somehow effect performance
4. Delete broken or unassociated registry values
5. Have their own type of defragmentation tool but does the same as the microsoft one (more or less)
Files just sitting about on your HDD has no impact on performance at all (unless there are thousands in a folder your viewing), the reason number 2 and 3 in the list is bold because it's the only thing you should do regularly that speeds up internet browsing, and most of the time the browsers themselves have an option to clear on exit anyway.
The worst thing to impact performance these days is to install programs that run themselves every time the computer starts, install multiple services, or programs that aren't well made; you can tell these because they use up more and more memory and CPU cycles for no reason. Most programs you install these days feel the need to install some pointless app that automatically starts when your computer starts; this in turn slows down startup time. I limit these as much as possible.
You don't need to be an advanced user to look after key things on your PC that effects performance, it's just knowing about it.
CCleaner (website) is quite a handy tool to have because this clears internet cache and temporary folders as explained above, it does have other tools built in (as most system optimizers [or cleaners] do): I admit that I too clean my registry but I think that's because I might have OCD or something, I like things to be "clean" like that, but technically offers next to no performance gain at all.
Managing startup programs and services are important too, as most programs (Windows by default enable specific ones after install) installs services you don't need. You can edit your startup programs via the registry, but i'd only suggest this if you know what your doing, technically a novice user doesn't
need to even know about the registry, instead, go to Start>Run (or press the windows key + R), type
msconfig and there you have all the tool you need to edit your startup items, by clicking the
services and
startup tabs, you can view what starts when your computer does; it's as simple as unchecking them.
NOTE: I wouldn't just go unchecking everything,
especially in the services tab, at least google it first.