Why do we need a question mark? Isn't it clear when a question is being asked?
Sometimes, questions aren't always noticeable.
If I would say to you, "You can," you would tell by the tone in my voice. However, if you're reading a book and see the phrase, "You can" with a period, you would just think it's that—a sentence without a question (declarative, in this case).
If I put a question mark in the phrase, "You can? then you would be able to tell that it is a question and not a statement.