10/3 wire run from panel to light switch?

[Eddie G]

I am looking to move some light switches and when I took the cover off the main wire running from the panel to the light switch box has 4 wires. Black, white, red and ground. Why would there be 2 hot wires running from the panel to the light switch?
Any help would be great!

Eddie G

 

[Irv]

This is usually for a "3-way" switch, IOW for a light that can be turned on/off from two different locations. Somewhere, probably on the other side of the room, there is another similar switch.