It's a religious exercise for the bombers. But what kind of god would welcome and encourage the destruction of what he had created? "Al-Qaeda claims responsibility in Iraqi Hotel Bombing," from the [1] Site Institute, with thanks to RB:

Furthermore, the message states that “He [the suicide bomber] entered to (the place) of the infidel whores, blew them up and completely devastated and destroyed them. Praise to God morning and evening.”