If Bulls re-sign LaVine, they’ll probably be looking to free up some cap space. This is a way the could do that while still keeping a first round pick, they move down but free cap space.
Mavs are also a team that needs to free cap space but a future trade down the line could do that. This deal gives them another guard if they have to trade Dinwiddie or lose Brunson, as well as moves them up 8 spots in the draft.