Butler joins the Lions after spending the first part of the year with the Chicago Bears. He was released in May and had been a free agent until he signed with Detroit on Friday. Previously, he spent time with the Pittsburgh Steelers, and he was actually their starter in 2012. (Butler, by the way, went to Georgia.)
In theory, the addition of Butler gives Sam Martin someone to compete with during training camp. In reality, though, Butler is strictly a camp body. His presence will lessen Martin's workload during training camp, and that's really the only reason the Lions added him to the team. Barring an injury, the starting punter job still belongs to Martin.
As for Lynn, he was let go after joining the Lions this offseason as an undrafted free agent. Lynn always seemed like a long shot to make the team, and that thinking was confirmed on Friday with the Lions parting ways with him.