Ideally, the Sixers will sign someone like Marcus Morris to start at the 4 position and play slim minutes--he'll be the starter, not the finisher.
The Sixers have basically no money under the cap. Martin was among the best players available at that price. To give us an idea how he is valued by the league, he was reportedly offered 4 years and $65M by the Heat. He left a lot of money on the table to try to win with the Sixers.
He's a competent, willing shooter; a good defender; a good per-minute rebounder; he can attack a closeout; and he can finish at the rim. AND he took a contract to play with the Sixers that allowed the team to stay a bit flexible with future signings.
This was a GREAT signing; about the best possible given their cap situation.