The Timberwolves already selected two PG's back to back with their fifth and sixth picks grabbing Ricky Rubio and Jonny Flynn. The drafting of Lawson obviously meant one of the three players were going to be shipped out. You could not possibly give all three of them the necessary playing time that is vital for rookies to develop.
The Nuggets had not had a first round pick for four consecutive seasons now, and will not have one next season either unless they make another trade.
Lawson will serve as the backup to star PG Chauncey Billups and could work his way into the starting lineup as a SG as he progresses.