I’m still baffled by compressor pedals, at least I can’t tell the difference when it’s engaged.
There are basically three reasons to want or need a compressor:
1) Dynamics Control.
Smoothing highs and boosting lows (volume-wise, not tone-wise) are what compressors do. But to use it for fixing sloppy playing never works, because you have to dial in too much compression for it to actually control anything, which squashes the signal adversely most of the time.
It’s much better to control the picking attack and use the compressor for more subtle smoothing.
Compressors add the “perception” of sustain by gradually increasing the volume as the instrument’s note levels decay.
But you will need some sustain to begin with (from both the guitar and amp) in order for it to sound natural.
Otherwise, like controlling poor dynamics, you’ll need so much compression that it will squash your signal as well.
This is how I use mine the most.
When a compressor kicks in (depending on how it’s set) some of the picking attack can be cut off or “rounded” giving a perception of having a “silky” “creamy” edge.
So, for me, a compressor is literally a “tone sculpting effect”, with any dynamics control and added sustain being only an additional side-effect to that.
All of this can be fairly subtle of course, which does make it difficult to understand what’s really going on at times.
So there can also be three other ways to look at it:
1) If you really don’t know why you need one, then you probably don’t need one.
2) If your amp already has great sustain and dynamics control isn’t an issue, then you probably don’t need one.
3) If you just desire a little more sustain, a bit of dynamics smoothing or a silkier picking attack, then a compressor may or may not be the thing for you.
Like I said, I use mine for “tone sculpting”, but I can easily live without it on the most part as well…