A small one for home studio use looks similar to this:
It lets you plug several mics, synths, and inputs from effects units etc, then outputs it into stereo. A little one like this gives you 8 channels, volume and pan control, basic EQing, monitoring etc.. plus phantom power for mics that require it.
Definately a requirement for any studio if you plan on doing some decent recordings. Even if just for the convenience of being able to leave everything plugged in.