basicly....

u have bass/drum_kick that needs more ooomph to it. so, u turn up the gain/level but then you expirence clipping...so u turn it back down, but it still sounds weak..
a compresser will let you increase that volume level, without clipping it out...
it's ALOT more technical then that and i gotta go home in 2 mins so not got time to write a lenghty one, but that's about the jist of it.
and yes, u need one!
there is a nice article in Computer Music this month on some basic compression stuff...