Snyder has played eight of his nine NFL seasons with the 49ers. He started 14 games for the Arizona Cardinals in 2012 before returning to San Francisco in 2013. He started four games with San Francisco last year and played in 14 of them, splitting time between left guard and right guard. Pro Football Focus ranked him 40th among guards in the NFL, giving him a strong run-blocking grade and a poor pass-blocking grade, which kind of makes you think he'll fit right in with the Giants' line.
It's unclear at this point how the Giants will make room on the roster for Snyder, but they need help at guard with Geoff Schwartz out with a toe injury and Brandon Mosley out with a back injury. They could put Schwartz (or, I guess, Mosley) on short-term injured reserve, which would require him to miss the first eight weeks of the season, and that would clear a roster spot. Or they could cut someone from the current 53-man roster to make room. The former option is more likely, but as of Tuesday night the Giants hadn't decided whether to make such a move with Schwartz.
Anyway, they need offensive line help, and this guy was out there and they apparently like him, so whatever. At this point, rookie Weston Richburg is slated to start at left guard in Monday night's season opener in Detroit and uninspiring veteran John Jerry is slated to start at right guard. So there are reps to be had at those spots.