Athletics Nation does a nice job of covering the trade of Khris Davis to the Rangers for shortstop Elvis Andrus:

As for roster fit, this is an obvious match for each team. Oakland desperately needed a shortstop for their contending team and now they have an established veteran. They also had a logjam of both right-handed bats and outfielders/DHs, leaving little room for Davis to get playing time and break his long-term slump. The Rangers were flush with too many infielders, but could use a big righty bat to help with a DH spot that posted the lowest bWAR in the majors last year.

AthleticsNation.com

The roster fit seems to be the biggest reason for the deal. Each player fills a hole, but each will need to improve on offense to really help each team win.