Have to agree somewhat with TK, drop D is a lazy thing for most people. It is sometimes used to just get a lower note in there to make the power chords sound crunchier. It may also be used as a drone if you happen to be playing in the key of D. Alternative tunings can be a lot of fun, but Drop D is definitely my least favorite, and as far as I'm concerned, the least creative alternative tuning. Stick with what you like the best and you can't go wrong.