Why would the Celtics sign Shaq?

Old or not, the Celts did make it to game 7 of the Finals last year. Shaq gives them a big body down low, and he can still play, not like he used to of course, but. Not, to mention it poses a match-up problem for teams like the Heat, who are short on big men outside of Z. I like the move, they weren't gonna get any younger this year anyway, and it's just a 1year deal.