In English, when a sentence begins with "Do you...", you immediately know it is a question because of that auxiliary "do", but Spanish does not have such an auxiliary, so you wouldn't know it is a question until you reached the end of the sentence and saw the "normal" question mark:
Quieres venir = You want to come
Quieres venir? = Do you want to come?
Also, while statements have the subject first (e.g. you) and then the verb (e.g. have), in questions the order is reversed: "Have you...?" In Spanish this inversion is common, but not compulsory, plus the subject pronoun is normally omitted, so both "you have" and "have you" would look like "tienes". In other words, again, unless you can see the end of the line to check for a question mark, you are clueless in most sentences about the kind of sentence you are dealing with. The inverted question mark provides this information in the absence of the clues you get from the intonation when you hear the language.
Also, it allows you to raise the intonation at any point in the sentence, rather than having to guess where.
