Major League Coach (MLC) duties are whatever is needed. Jose Hernandez has been the MLC for the O's the last few seasons and he did a little of everything. Threw batting practice, oversaw defensive drills, etc. Kind of a general assistant. Herandez also spoke Spanish so he helped the Latin players.
Buck's duties may be different. Robinson is Spanish speaking.
Buck probably has the most management experience of the coaches. But in order to be ready to manage in the majors coaches normally spend some time has bench coaches. He is a step away from that.
Interesting that there are 12 coaches. Weaver had 4.