Rachel - I agree with the others, but some people need to use a light as a means to supplement/increase the ambient temperature of the viv.
For example, because it's a little colder in the winter where I live right now, I find that I need to run a 75 watt red bulb on a 24x7 basis, because the under-the-hank heat mat on the "warm side" of the tank does not keep the temps high enough. I won't need to do this during spring and summer.
For example: Depending upon the under tank heating source, it is possible to crank up the heat mat/tape so high (to increase overall temps) that the bottom glass gets way too hot and could burn the snakes belly (and they won't really know its burning). I counteract this by keeping my heat tape at just the right level (85 degrees) via a thermostat (rheostat/dimmer is good too), and supplement the overall ambient temp with the red bulb.
With this setup, I am running about 75 degrees on the cool side, and 85 degrees on the warm side. My viv is somewhat large, so smaller vivs could probably use a 50 or 60 watt bulb.