This was the one head scratcher for me, but it was 51, so whatever.
Reportedly the Celtics were looking for a player that would agree to a two-way contract at the pick (which makes sense). Not every player would want to agree to that prior to the draft, so that presumably limited their pool of guys to pick.
On top of that, his defense is NBA-ready, so if you have to play him due to multiple injuries and illnesses at the position, he can at least give you something playable at one end of the court.
I don’t expect he’ll have a long and storied career, but you never know. Right now he’s someone to develop for a year in Maine, have available in case of emergency, and then see what you’ve got. In a perfect world he’s a smaller version of the PG that Avery Bradley never became.