Barnes entered the summer as a restricted free agent, but the Warriors renounced his rights after Kevin Durant agreed to join them. The unrestricted Barnes quickly met with the Mavericks and verbally agreed to terms after Dallas had planned to offer him a max-level offer sheet on July 7 when the NBA's moratorium was lifted.
Barnes is a 25-year-old forward who started for the Warriors as they won 73 games and went to two consecutive finals. There's no doubt he's a capable role player, able to play the fifth starter role. However, in Dallas, the Mavericks will ask more of him. That's why they signed him to a max contract, after all, believing he could grow into a larger role.
So far, there's not a lot of evidence that Barnes is capable of that. However, he's young, athletic and by all accounts, a hard-worker. Under Rick Carlisle's tutelage, you'd like to think that Barnes can develop his game even further than he has so far. Barnes is also a good defender capable of defending multiple positions right now.
For the Mavericks, it's strange that they were willing to offer Barnes a max deal and not Chandler Parsons, who is a much more capable scorer in almost every area. But when Dallas chose not to offer Parsons, they still believed they could add Mike Conley and Hassan Whiteside together. By the time they offered Barnes this contract, there was really no other options left on the board.
It's notable that reports from just a week ago indicated that the Mavericks were not interest in Barnes at a max contract. If there's anything besides necessity that changed their mind, that will be interesting to see.
Either way, at least the Mavericks now have a starting five: Deron Williams, Wesley Matthews, Barnes, Dirk Nowitzki and Andrew Bogut. That's more comforting than where they were yesterday.