Boiman has been in the league since 2002. He played for Jim Schwartz with the Titans for the first four seasons of his career, which is one of his connections to the Lions' current coaching staff. After leaving Tennessee, Boiman moved on to Dallas, but he was released during the final round of cuts. He was quickly picked up by Indianapolis, though, and he played in every game for the 2006 Colts, which won a Super Bowl. He again played in every game in 2007 and then signed with the Eagles the following offseason. Like his time with Dallas, it came to an end during the final round of roster cuts. Since being let go by Philadelphia, Boiman has been with the Chiefs, Titans and Steelers.
During the course of his career, Boiman has not started a ton of game. His most starts in a season came in 2008 with Kansas City when he had nine total in 11 games. The most starts he made in a season prior to that was only seven, so I would imagine this is a move for depth and depth only. While Boiman has showed his talent at times during his career, let's not forget that he hasn't been able to stick with teams for an extended period of time ever since his first stint with the Titans.
To me, this is quite simple: Detroit needs added depth at linebacker and Rocky Boiman's role will be to help provide that. Will he make the team? It's tough to tell considering he will only have one week of practice before the final cuts are made, but helping his case is that Schwartz and Cunningham have some previous experience with him.