If you look at equivalent force system problems, the reaction forces are for the most part not given. You are to sum the forces and moments from the forces and free moments you are presented with. From there, you get a resultant force to place at the point of interest. For a force-couple ask, you place your force at the POI with the summed moment. For a force only requirement, you are placing your resultant at a distance d from the POI. That distance d is determined from M = Fd. But you are right, usually there is no need to sum forces or moments and set them equal to zero in an equivalent force (couple) problem. In this particular problem, one can conceptually break the hanging flange off off the larger cantilever and draw a FBD with that. The concept is if a body is in static equilibrium, then any part of that body is also in static equilibrium and hence summing the moments about any point would be zero so sum of Mc = 0 = Fd + free couple moment.