The Browns went offense with their first two picks -- Justin Blackmon and Kendall Wright -- so it makes sense they'd focus on defense with this one.
SB Nation's Browns blog, Dawgs By Nature, explains the move:
Linebacker Scott Fujita has ended the past two seasons on the injured reserve. While he has been a veteran to rely on when he does play, the Browns desperately lack depth at linebacker and could use some youth to inject some more life into the group. His quickness and explosiveness should help the team in pass coverage, as the team had trouble covering tight ends at times last season (see Jared Cook outrunning the linebackers for an 80-yard score).