Decline bench actives more chest muscle fibers than any other chest movement. It does this by taking the front delts out almost completely of the picture. Flat bench is a good mass builder, but puts a lot of emphasis on your front delts.
A good chest routine will incorporate flat, incline and decline presses for total overall chest development.
In my own routine, I normally hit decline barbell second after flat dumbell presses. That changes every few months of course, but i have found it to be very effective.
I think it was Ken Waller (Back in Arnold's day) who always felt that his delts were taking the brunt of his flat bench work, so he cut them out all together and started doing declines instead.