Sometimes, a player acquisition doesn't have to add legitimate value to make legitimate sense.
According to Marc Stein of ESPN.com, the Boston Celtics are close to adding journeyed, hyper-athletic big man Sean Williams for the remainder of the season. Williams isn't likely to pick up much playing time on a team so dedicated to being on the same page, but in terms of reserve bigs to have lying in wait at the end of the bench, Boston could certainly do far worse (and in a way, already has done worse with the signing of Ryan Hollins).
Kevin Garnett, Brandon Bass and Greg Stiemsma are—and will be—Boston's functional frontcourt, barring foul trouble or injury. Yet NBA coaches and general managers need to be prepared for just such contingencies; although Garnett and Bass generally keep their fouls down and the trio as a whole has managed to escape the fate that befell the injured Jermaine O'Neal (who will be waived to make room for Williams) and Chris Wilcox, there's no telling what the coming weeks might hold, nor when a player like Williams may hold more than practice value.
Of course, should it come to that, it likely won't matter that Williams is notably more effective than Hollins; the more Boston is forced to lean on their emergency reserves, the more readily apparent the weaknesses of those players will become.
As a result, the net value of an acquisition like this one is relatively minimal, particularly if Williams is asked to play anything more than spot minutes, or asked to be anything more than a stopgap for when Stiemsma picks up a few quick fouls. In that way, Williams' signing may be a mere postseason formality, but there's no reason to scoff at the Celtics for fortifying their ranks. Williams can move within a defense and block shots if nothing else, and considering the alternatives both on the roster and on the open market, they're wise to pick him up.