Overhead luggage is the only place you can take anything fragile. You have no control over how your checked baggage is handled and anything you have to take with you into the cramped seating space will get squished.
For boarding it might speed things up but often boarding is done before all checked luggage is loaded so it will probably not let you take off faster either.
Similarly, for takeoff I agree that it wouldn't necessarily save time, net. But it would help with the frustration of standing in line, backed up on the jetway while everyone is struggling with setting up the initial conditions for the deplaning scenario I described above. At least people could get seated sooner and be comfortable for longer while they're waiting for takeoff.
[1] Based on a true story.