That is to help you identify the case.
This explained it to me rather nicely.
I agree with Hohenems. I will add that although there is a common belief that you cannot end a sentence with a preposition, there is no such rule. (See http://www.quickanddirtytips.com/education/grammar/ending-a-sentence-with-a-preposition?page=2 for a list of numerous citations.)
I'm a 45 year-old, college-educated, native English speaker (American) and am unaware of any proscription on using which to refer to plural objects. I cannot think of what word could possibly be used instead.
"Which apple is yours?"
"Which apples are yours?"
In both cases, I'm asking someone to identify specific items that belong to them. In the first case, I'm expecting or assuming there's only one item. In the second, I'm assuming that two or more apples belong to you.
Both are most certainly correct in colloquial usage. Unless there is some arcane rule with which I am unfamiliar, they are also grammatically correct in formal, written context as well.