Sep 24, 2004 09:29 am Preamps are used to boost microphone signal to be stronger and more acceptable by the recording device (that's the non-techie explination). If you have a single preamp unit, and record one instrument at a time, a mixer is not necessary, but it can certainly be handy to prevent constant cord switching and crap...plus, mixers have preamps, while not the best preamps all the time, they definately get the job done for many people, myself included.
There are a couple articles in the "computer recording" category of the tips section regarding different ways to use mixers, I suggest you read those for more info.
Welcome to HRC.