It limits the user error pens cause. You can look up old disputed ballots, many of them are things like; people sloppily starting to fill out A then realizing t

It limits the user error pens cause. You can look up old disputed ballots, many of them are things like; people sloppily starting to fill out A then realizing they really want B, making such a large checkmark that it covers A and part of B, or those pranksters that fully color in A B and C. As this shows, it still allows some user error, but they hope for less.