There has to be some way to align the drum with the front roller and the reel, so that the whole mower is square to the ground, or you will end up with uneven cutting results as the front roller bears on just one corner, and digs into the lawn by varying amounts. Think of the drum adjustment as a one-time setup adjustment to make your mower sit flat on flat ground. If the factory had perfect tooling and robot assembly it would not be needed, but in the real world of cheap low-volume tooling for a small factory, it's usually cheaper and easier to provide an alignment adjustment. Things are different in the automotive industry, but in cottage industry you rely on skills rather than systems.

Ross' comment makes sense to me: you might as well put it on the side of the mower where drive chain tension will not be trying to shift the adjustment all the time. That is, I suggest you put the adjustment on the opposite side of the drum to the drive chain sprocket.