As multiple sources have reported and confirmed, the Brooklyn Nets and Atlanta Hawks have agreed to a deal in principle that would move Joe Johnson to Brooklyn and an assortment of expiring deals back to Atlanta—all of which function as a precursor to Deron Williams' decision regarding his possible future with the Nets.
In that regard, Brooklyn's motivations are obvious; they've re-signed Gerald Wallace and agreed in principle to add Johnson, all for the sake of giving Williams a competitive core to work with. Those two may not be All-NBA talents, but they're fringe All-Stars with versatile skill sets and proven track records. Add in Brook Lopez, and the Nets have a fairly promising core—provided Williams agrees to re-sign and the Johnson deal pays off.
For Atlanta, this is a move all about future flexibility, to which Johnson's unbelievably massive contract was a clear hindrance. Without Johnson clogging up their cap picture, the Hawks have their choice of in-house players to build around, along with the cap space to chase one of the major players on next summer's free-agent market.