Purpose of the pre-sentence punctuation?
Why in Spanish are there upside-down punctuation marks at the beginnings of sentences?
4 Answers
The question you should ask yourself is "Why in English are there no upside-down punctuation marks at the beginnings of sentences?"
In English, questions usually start with question words (interrogatives) - Why, When, Where, Who etc but that's not always the way in Spanish. In Spanish a statement can be made into a question simply by adding the question marks around it.
Va a la tienda. You are going to the shop.
¿Va a la tienda? Are you going to the shop.
When you are reading outloud, to make sure that you speak with a questioning tone to let the listener know that you are asking a question rather than making a statement you get a helpful little upside down question mark right at the beginning 
Nifty aye!
And if you are just reading to yourself or writing something down for someone else it's the same principal.
http://prayerfulhomeworkers.com/images/question_mark_3d and upside down.PNG
I like it because it lets you know what you are getting into before the sentence starts. If it is a long question, you know you will be asking in the form of a question from the start instead of having to all of the sudden intonate your voice to form the question. The same goes for exclamation: I automatically know if I need to be excited or start yelling before the sentence even starts.
The punctuation marks are used to signify the beginning of the question or exlamation for clarity.
In written English, questions like 'You like cookies, right?' and 'When I leave (the house), should I turn off the lights?' can be a little unclear as to what the question is and where it starts. In written Spanish, the beginning punctuation makes it very clear.
The sentences above would be: 'Te gustan las galletas, ¿no?' y 'Cuando salgo de casa, ¿debo apagar las luces?'
Hope this helps! 
