Why?
If you've got the room for a 128 channel full-size desk, then go for it! For the rest of us mere mortals, we've got to use normal-size gear and that sometimes means mixing things before sending to the final destination. In my case, I had a Layla 24/96 with 8 ins and 8 outs. But only the main left and right out of the Mackie CFX16 went to it (I used the others for different reasons). So, even there I had a "sub-mix" if you will.
This whole subject deserves it's own complete topic.