The hole is to access the bolts that hold the bearing retainer and backing plate to the axle housing.
This is why. There are supposed to be plugs in them but usually once they are taken out they don't get put back in. When you have to remove an axleshaft, you'll be happy that hole is there.
1977 Cj5, 258, T-18, 2.46:1 Dana 20, 2.5" BDS lift, OBA, 38.5's. Chevy 60 frt, 14b Rear, Full hydro, 102"wb, full DOM cage.