This is a pretty standard kind of move in the Belichick MO. He's a guy you probably don't want as your 17 week starter at either tackle spot. But he's good enough that if you HAD to do it, it's not an automatic fatal weakness. So you go into the draft hoping to pick someone better who will win the job, but if you get to pick 14 and either A) The best tackles are already off the board and the value isn't there, or B) There's a REALLY good player at another position that you don't want to pass up, say at CB or WR, you don't feel like you're handcuffed into taking a tackle.
So best case (barring other OT signings): You draft a LT at 14, Brown moves to RT, Anderson is a top end swing tackle.
2nd best case: You draft a RT, either at 14 or with a small trade down, Brown stays at LT for one more year and Anderson remains that swing guy.
Worst case (from an OL perspective): Anderson and Brown are your week 1 starters, maybe you've got a kid from day 2 of the draft who needs some time to season, but you've also added a top player at another position of need at 14.