From PPG article about his signing:
The Penguins don’t really have a job open for him, although he could conceivably compete for fourth-line time in training camp. A much better way of looking at the signing: It provides the Penguins with a player who has suited up for NHL games, should injuries occur.
Unless there is a bigger trade coming, it still doesnt make too much sense. It is not a 2 way deal, so they wont send him to WB/S, and I doubt he would have signed with pens if he was to be healthy scratch most of the time. Maybe pens were only team that offered him a contract and he had no choice but to to sign, who knows.
The depth part makes the most sense. Gives the Pens an insurance policy if one of their centers or PK guys isn't healthy to start the season and then gives them depth whether he's sitting in the pressbox at the NHL level or a quick call up playing a big role for WB/S.
Otherwise, why would a 1-way deal keep them from sending him to the AHL? There's a decent handful of teams that have one or more (or 5 in the Leafs case?) guys on league minimum 1-ways that pass through waivers and go to the minors. If anything, the 1-way deal might be protection from him getting claimed on waivers by a team looking for a quick fix at the beginning of the season and won't want to pay his one way if the he eventually clears waivers for the claiming team.
As for his other offers, I would imagine him being a center that can either skate on an NHL 4th line or tear it up on an AHL 1st line he got quite a bit of interest, but why he settled on the Pens is a curious case. I guess it's possible the Pens were the only team that offered a one-way? Although that doesn't make much sense considering the Ducks gave an older and less NHL useful center in Ben Street a $750k one year one-way deal... Maybe Grant became good buddies with Gonchar when they were teammates for a couple months in Ottawa during the 2012-13 season and he wants to be coached by him?
At the same time, maybe he got enough interest to where he's pretty confident a team will put in a claim on waivers, but first wanted a shot to stick with a contender this season. And if he doesn't get claimed, at least he doesn't lose any salary playing in the AHL.