if you have 24 mono chanels, you cant have just left and rigt out, thats why you can have 4 busses or subgroups, so you can say what mic goes out from what out or bus or subgroup:
the 8 mics of the drums bass and guitar bus 1 & 2
the lead vocal, backvocals and lead guitar bus 3 & 4
and then main out or master...
you can use the for multitrakc recording, for mixing, they bring you a lot of help...