A fan/light combo when you want to use a dimmer for the light. I installed two fans in my house. One room had the red wire, so I could attach a fan with a light kit, and use the dimmer for the light without the fan speed changing.
In the other room, I did not have a red wire. I tried to hook it up with a dimmer and the dimmer controlled everything including the fan speed. So.. when I dimmed the light, the fan speed went down no matter when setting I had the fan on by the pull chain. I changed this room back to a regular wall switch and live with it, rather than burning out the fan motor. -Steve