The Buffalo Bills value versatility in their reserve offensive linemen. If you can play multiple spots up front, then it makes you eminently more valuable to the Bills. Especially considering that carrying fewer offensive linemen on the 53-man roster opens things up elsewhere, this makes sense.
The Bills cross-train multiple players throughout the course of their offseason work. Sometimes, these experiments go well and the players find that they can handle multiple positions. Sometimes, the experiment doesn’t go well and the Bills find that they need to look elsewhere. Of course, there are also times where a player is so good at one position that it becomes silly to think about moving him.