It's obvious that the Cavs are not contenders. And as they are built, it will be difficult for them to be. And there are rumors that Mitchell leaving is a matter of when, not if. Mobley has yet to establish himself as at least a borderline all-star. Having a wing with good size, who shoots well from three will help a lot with the spacing of their offense.
The Nets need an extra franchise player. Bridges alone is not enough. Hence Mitchell. He's not top 5 in the league but he's better than Bridges.
This trade strengthens the Nets and gives the Cavs the opportunity to calmly and without much pressure from already being a top contender, focus on developing Mobley as an all-star. Garland is a good PG, both in scoring and in playmaking. With patience they could be a very good team to watch.
I see this trade as a win-win.