Why Electronic Drum Sets?
There are a number of reasons for it. Hopefully I can satisfy your curiosity with one of the following scenarios.
One could be that the drummer uses them as a practice kit at home. Limited space, neighbors, and convenience would account for one having an electronic kit.
Another, the electronic drums often times are ran through a midi and an effects loop. The drummer can give any drum virtually any sound he could possibly desire. Anything from a rain drop falling in a bucket, to a car crash, or explosion. That can add a lot of dimension to a song.