I get asked this question a lot. I just answered it recently and thought I'd take a proactive step and post it.
Mixers are more than knobs that let you vary the volume.
1. They let you vary volumes without shaking the camera or getting in the way of the camera op.
2. You may need to do that a lot with some people. I ride gain even if one person is talking if their voice fades on the end of each line. You can only do this in a relatively quiet environment, otherwise you bring up the ambient noise.
3. Mixer preamps (good ones) sound better than camera preamps.
4. Good mixers have input transformers that scrape off RF before it get into your audio.
5. Good mixers have limiters that allow you to record hotter, keeping your audio further above the noise floor without distorting.
6. Good limtiers have EQ that lets you roll of LF HVAC noise before it gets into your audio.
7. Good mixers have mulitple outputs so you can feed more than one camera, or separate recorder simultaneously.
8. Good mixers make your sound better. If they didn't pros wouldn't use them.
9(a) Really good mixers allow you to monitor the recorder via a headphone return, so you can hear any problems happening between the mixer and the recorder (eg. line inputs accidentally switched to mic level).