It may be used in future updates, last year they added a few extra rules and wepon types. Maybe that later you can add a sheild type building onto a attack building that gives some protection but less than the defence tower? This is just my theory.
@Kortgat is correct. This is to keep our options open for future updates and implementations of those updates. You are not missing anything at this stage in the game, one cell will only ever have one building as the rules currently stand.