I too was baffled by compression. I was also baffled by the singer/guitarist in my duo (an experienced FOH/sound tech) missing compression in our sound, applying that compression and being satisfied when I couldn't hear the difference. "If you can hear a difference in tone then it's too much compression," she would say, I couldn't get my head around that. Thankfully I am acquainted with the excellent @Osiris, who spent a day demonstrating and educating me in the subtleties of compression, that lead to me understanding the subject a lot more and, eventually, to acquiring an optical compressor, much more suited to the type of music we play. Now I can't do without it, I notice when it isn't on and yet I'd still struggle, really struggle, to explain exactly what it's doing. I use a very expensive preamp and the Effectrode PC-2A Compressor, given the choice between the two I'd keep the compressor, the preamp is tech, that compressor has some kind of juju I can't do without.