This will fatten up the sound and level off the peaks and valleys making the bass easier to bring up in the mix. I prefer not to use a lot of compression on instruments unless it is absolutely necessary. Vocals are a different story. Singer's voices fluctuate continuously and compressors can even this out. If the singer then hits a screamer, the gain reduction is already happening, but does not give you the pumping or breathing sound.
Attack time is fairly quick.
Release times a little slower. Putting compressors on inconsistent singers will make your life easier and their sound much better. Protecting your PA gear is very important. There are a couple different ways to do this. I prefer the latter of the 2 described below.