I don't really understand why you would...
He's a $15M expiring contract.
Financially, it makes no difference to us if we buy him out or keep him, come the offseason he comes off our cap anyway.
Having him on our roster gives us another backup big in the playoffs, which (based on Celtics injury history in the past) isn't a bad insurance policy to have given he's still quite productive as a player.
Why we would buy him out, I just don't know. The only benefit I see to it is that it to Lee himself (he would have a chance to get on to a Playoff roster this year), or maybe if it allows us to eliminate a potential cap hold in the offseason - then fine.
Otherwise, doesn't make much sense to me.