If I have a Yamaha MG12XU mixer, why do I need a stereo mixer in the rack?

Perhaps I do not need one of the expanders for the FH2. The idea was to be able to play polyphonically and also have some slots leftover for random lfos etc. But I'm sure 1 expander would be good. If I drop one of those any suggestions for another 4hp module to fill the case?

I'm a big fan of Trent Reznor and his experimental scores so this will heavily be a sound design rack...however I want to be able to play live with my keystep pro. Bitbox for percussion, keystep pro sequencing that. Then I have many voices leftover in my rack.