Yep, Tekker's right. If you record a snare, for example, with no compression, you'll have to keep the level pretty low to compensate for the associated peaks. So, when you go in with a software compressor afterwards to control the peaks and boost the overall signal, you're going to boost all of the noise in the signal along with it. If you had used a compressor when tracking, you get a hotter signal and are capable of achieving a much better signal to noise ratio in the end.