Using a mixer will save you repatching instruments for input to your DAW/ Audio Interface.
It should also allow you to integrate outboard gear (HARDWARE) into your setup using inserts/ sends.
A mixer may have useable EQ and FX, handy for studio or live work.
A mixer will help you to route audio signals to you MPC/ sampler from a variety of sources effortlessly.
A real mixer can be used without a mouse or a computer.
If you don't know why, exactly a mixer is necessary, it's probably because
YOU DON'T NEED ONE.
http://www.tweakheadz.com/choosing_a_mi ... tudio.html