Why do some video cards have 2 DVI outputs?

Is the only reason to output to 2 separate monitors? Or can you use both outputs to power the one monitor to gain higher resolutions?|||the only reason is to be able to connect two separate monitors.|||only the former, not the later. 2 monitors, not higher resolution.|||2 monitors sometimes 4 if the board supports that many in which case you would use a cable that splits each output into two.