Matrix, What's the point?

Really diving into the Wing Rack was watching Drew's (yes we're on a first name basis) video on main bus sends to a matrix. I'm doing the editing in the laptop app right now to not disturb the family. What is the true benefit of matrix vs aux fed sub particular with the wing?

Also, how does the signal flow work, should I leave the channels on in the main bus at unity and adjust only the main mix? Leave them at unity in the main sub bus and the sub bus matrix? Should I adjust only the matrix main?