This is another instance of a mystery of which I've only recently become aware: The (mis(mis))use of grep in place of the (mis)use of map in place of a for-loop.

Definitely agreed that it's a strange (mis|ab)?use of grep, but the map variant seems pretty canonical to me - it transforms the elements of a list and stores the resulting new list. The only thing that strikes me as questionable about it is that it overwrites the original list with the new one.