depending on the model you buy you may be able to:
even out picking dynamics
use as a squishy effect in itself
add nice colour to tone
make dry sound punchier by blending in a compressed signal
increase sustain without using OD/distortion
increase sustain combined with OD/distortion
boost signal to drive pedals/amp, or any other use of a boost
post regularly on forums asking which compressor is better than the one you have now..
A compressor is an "always on" pedal for me. I agree with a previous poster about the comp making it seem that your amp is turned up more than it is. That's how I hear it. Keep in mind that I don't play a lot of chords.
Great video showing the various uses of compression.
For me, it’s an indispensable tool. I leave it on 100% of the time and it significantly contributes to my core tone. I dig making my guitar sound massive without using fuzz or distortion, and compressors are amazing for this purpose. Andy Summers is the king of this approach I run single coiled strats into Fender amps, and I find your choice of guitar/amp severely changes the way a compressor is going to interact with your set up.