The Patriots traded for Texans wide receiver Keyshawn Martin and waived wide receiver Chris Harper.
Why?
The simple answer is that he's just a better option than Harper right now.
The Patriots waived Harper and no team claimed him, meaning that he has a space on the practice squad if the team so chooses. Harper needed time to develop as a route runner and he'll have plenty of work on the practice squad.
Martin isn't going to be a major starter in the offense, but he offers incremental improvements over Harper in every facet of the game.