you probably aren't a diehard fan or you'd know most of this...they started out synthy, point was to have no drummer(or much guitar) live and in the studio..long story short, they decided to "grunge" it up a bit in the mid 90's for the first time with live drums live and in the studio(Songs of faith and devotion was the first introduction of live drums on an album and the tour had plenty of live drums)
Depeche have always been non traditional in the roles of the bandmembers, for most of their career they were a four piece. Martins the songwriter(writes most songs on guitar or piano mostly) daves the charismatic frontman, alan's the studio wizard producing most of the CD's and translating martins sparse demos to the electronic masterpieces they are, and fletch is basically martins best friend from childhood and basically helps manage the band and fake keyboards live.
anyway, alan did some drumming on the songs of faith and devotion tour(again, since the cd had real drums for the first time), left after that tour. They replaced him with basically different producers for every subsequent CD, but decided (probably based on the mixed fan reaction to real drums) to continue more with the classic depeche sound...however(I'm sure of dave's insistence, who has always been trying to stear the band in a more "rock direction") the live drummer has become a staple of all subsequent tours, perhaps also to give a different dimension to the older 80's material..
anyway, hate to be long winded, hope this makes sense and helps, hell, there are like 400 page books on the Mode, hope this wasn't too long winded