Agree with B3 here.
And don't underestimate the freedom of changing sounds separately. Say you have sounds A,B,C on your keyboard and you want to combine them with 1,2,3 on your module. If you did not have the module you'd have to switch from 'play' to 'performance' mode to combine them as you wish, or you'd have to pre-program 3*3=9 performances on your board.
If you have an independent sound source you can mix them as you wish while not having to stop playing. A lot of times I keep playing with my LH while the RH changes sounds on the module, with some experience you can do it quite smoothly.