to clean out all the files that slowed it down.
There are no such files in OS X.
Normally, your Mac should run fine and not need any regular preventative maintenance. There is no reason that OS X will degrade its performance over time, or as the result of accumulated files.
I would concur that reinstalling is rarely necessary, unless you have deleted or modified some essential system files. Otherwise, nearly every problem is solvable without reinstalling the OS.
Also, I would stay away from any "cleaning" apps that are often advertised, as they are at best unnecessary and at worst, harmful.
If you do get problems with your Mac, there are a number of things you can do to restore performance, most notably emptying the caches. A corrupt cache might cause slowness.
Note that caches should not be regularly emptied, as the whole point of a cache is to speed things up.
I used to reformat my PC’s hard drive monthly
Wow. Welcome to OS X. I'm surprised your hard drives didn't fail after a couple of years of doing that.