Because some people want the best of 2 worlds... they want 88 keys and a superb piano action and top of the bill synth performance (Motif/M3) and an arranger... this is where modules shine..

Or they want to add an arranger to their digital piano at home. Arranger piano's are not on par with totl arrangers.

Or they want to add Audya styles to their great sounding PA2X or T3. but not use 2 keyboards on stage..

A friend of mine is using an 88 key Oasys combined with a Roland V-synth and a Wersi module as a OMB (and a PC for VSTís), but also uses this stuff in the studio. He has absolutely no room for a keybedded arranger on stage as a 3 piece setup would make him loose the contact with his audience. (he has setup his system like a V, with V-synth to the right and wersi to the left.

Lambert is most of the time using a Ketron module next to his Wersi organ, its a great addition to the Wersi sound.

How many more reasons do you need for modules?