A quick way to get 'drop-E' tuning is to clamp the capo across the top 5 strings at fret 2.

Also once you are used to it, playing acoustic guitar in horn-based bands just seems easier (I know, the two don't really mix all that well!)

As Kevin points out, it can also lend a different timbre to a song. Some certain sweetness in open chords on 5th-7th frets that you just can't get by fretting alone.