When you are a marginal player you are looking for any chance to prove themselves. Getting multiple game time kicks as the temporary punter might lead to a full time job on another team. Injuries happen.
That’s very likely why he was willing to sign on with the Niners for a few weeks.
Harvin was one of the top punting prospects coming out of Georgia Tech, having won the Ray Guy Award (given to college football’s best punter) as a senior, and actually getting drafted by the Steelers – sure, it was only as a seventh-round pick, but most punters aren’t even drafted.
He played for Pittsburgh for the past three years, but apparently struggled with consistency, and he was released after last season. He went unsigned during the off-season, and likely saw this short-term gig with the Niners as a way to hopefully do well, and re-establish himself as an option for a team which decides to (or needs to) sign a different punter during the season.
It’s really the only way to try and stay in the game. Teams don’t keep kickers on the practice squad.