The Celtics ranked fourth in defensive rating last season, and Howard's presence would take them to an even higher level. Howard is still one of the NBA's best rim protectors and shot blockers, and he'd likely improve individually playing behind the likes of Marcus Smart, Avery Bradley, and Jae Crowder.
Howard's interior defense would also be beneficial against Eastern Conference behemoths like Andre Drummond, Jonas Valanciunas, Nikola Vucevic, and Myles Turner.
The Celtics might've had a tremendous defense last season, but they ranked 25th in defensive rebounding percentage. Howard would provide an immediate boost, since he's still an elite rebounder.
Howard would operate as a mistake eraser in the paint and finish a chunk of possessions by securing rebounds. Those skills alone would help the Celtics.
Howard has never really bought into being a rim runner/roll man in pick-and-roll situations, always insisting on being a classic low-post scorer. But if he does adapt to the modern NBA and become more of a rim runner, he'd personally benefit as a player—and he'd help the team he signs with.
"This will be my last chance for a really big contract. If I am utilized the right way, I know what I can do for a team, a city, a franchise," Howard recently told ESPN's Jackie MacMullan. "I know my worth, and I will bust my butt this summer to show my worth to everyone.''
Howard is capable of finishing lobs athletically, making him a potential great pairing with Isaiah Thomas. The mere threat of that would force defenses to collapse into the paint, giving spot up shooters a little more space to fire away.
There's also the salary factor for the Celtics. Assuming they don't sign Kevin Durant, they'll have approximately only $40 million in guaranteed salaries. For teams that don't reach the minimum salary threshold (about $85 million this year), they must pay a "penalty." The punishment for falling short of the salary floor is that teams get charged up to that amount, and the excess amount is divided amongst the existing players on the roster.
While that "penalty" isn't that big of a deal, since it'd operate as a bonus, signing Howard to a two-year maximum deal with an opt-out in 2017 (about $30 million per) would move them closer to the minimum while giving them a great deal of flexibility. Not only would Howard help the team this year, but his non-guaranteed contract for 2017 would allow them to use him as a trade asset leading up to free agency.
One potential approach for the Celtics this summer is to essentially do nothing and roll with a young roster. But as it stands, Kelly Olynyk and Jordan Mickey are the only bigs on the roster. They need to add someone to at least remain competitive, and there would still be plenty of minutes available for the young players.
Dwight Howard doesn't need to be Superman anymore. But he still has plenty he can offer in the form of rim protection, rebounding, and rim running.