Because he is exactly the sort of player who can catapult the Bucks all the way to the 2004 Final Four!!!
Seriously, it's a small but necessary move. Keith Bogans has a shot to inexplicably inflate his value...somewhat. He's going to get time as a 6th man, protection of Charlie Bell sort of a dude. This is the Derrick Byars response. Not bad. But some marginal upside.
He's an upgrade over Tyronn Lue. He never lead a team to an Elite Eight!