Leon Washington is mostly known for his kick returns, which would be the position the Bucs would most likely want him for. He led the NFL in kick return yardage last season and has been a more than capable punt returner throughout his career. The Bucs, meanwhile, have not had a capable returner of punts or kicks since Clifton Smith left the team due to injuries after the 2009 season.
The fact that the Seahawks would rather release Leon Washington than trade him the Bucs is a bit odd, to say the least. Perhaps they did as a favor to the running back, who may prefer to return to his home town of Jacksonville. Or perhaps he simply doesn't want to play for the Bucs, or the Seahawks had a different reason for declining the trade.
Washington is a free agent now, so the Bucs may yet to try to sign him.