People need to have a mixer to when they have lots of MIDI synths, mics, or want to use quality outboard gear like reverbs, delays, compressors, pedals and other stuff that needs to be routed to the computer. If your mixer has enough channels, you can also route your audio interface's outputs to the board and use the EQs and sends/returns to touch up and polish the sound in the analog domain. This allows you to separate vocals, drums and bass so you can have a real hardware fader to move when mixing. If you have an 8x8 audio interface then you can have 8 separate channels at the board. A pro might have 24 channels streaming from the computer. With a mixer, you gain flexibility, ease of patching in gear, and for many of us, it's just more fun to have real faders and not stare into the computer screen.
Courtesy of TweakHeadz (http://www.tweakheadz.com)