There's a good chance Buster Posey goes to Triple A Fresno soon - at least for now. But the 23-year-old catcher is one of the best prospects in the game, so the Giants figure to call on him at some point, even if he doesn't make the team. That would be determined, in part, by Posey's play in the minors, but performance wouldn't be the only consideration for the Giants.
If they keep Posey, who has 33 days of major league service time, in Fresno until May 20th or later, he can pick up a maximum of 136 days of service time this year. That would keep his service time total under the 172 threshold days players need for a full season. This means the Giants can call on Posey in late May and be confident that he won't have 172 days after the season. That would postpone his free agency until after the 2016 season. Bottom line: the Giants can delay Posey's free agency by a year if they call him up in late May.
If the Giants are happy with Bengie Molina's production at the end of May, they could keep Posey in the minors for another month and prevent him from becoming a Super Two player. If the Giants call on Posey at the beginning of July, he won't have 130 days of service time at the end of the season. Bottom line: the Giants can save themselves money in arbitration if they call Posey up at the beginning of July.
This isn't to say that the Giants will make roster any decisions based on Posey's service time clock, but the options above figure to be a consideration for the team, as they would be for most clubs in this situation.