It's more likely than not that Graham Glasgow could have received a better contract offer from another team on the open market. To Glasgow's credit and the Lions credit, the two parties worked out a new contract that keeps Glasgow in the Lions fold.
The value of Graham Glasgow far exceeds what he provides the Lions at either Guard position. Glasgow is a much needed quality insurance policy for Frank Ragnow at Center, knowing Ragnow will very likely will continue to be kept out of one or more practices each week leading up to the next game, for maintenance purposes. Glasgow is also a great Lions culture fit.
Unless the open market for Jonah Jackson doesn't materalize to his satisfaction, it's likely that Jackson will be signing elsewhere. Hence, retaining Glasgow is that much more important.