Why does Spanish use two question marks (¿?)

Spanish is the only language that uses a beginning question mark (¿) and a beginning exclamation mark (¡)… but why? A few examples will answer the question:

-Sabemos cómo lo hacemos / We know how we do it.

-¿Sabemos cómo lo hacemos? Do we know how we do it?

-Tratarán de terminar pronto / They will try to finish early.

-¿Tratarán de terminar pronto? / Will they try to finish early?

-Todos fueron a París / They all went to Paris.

-¿Todos fueron a París? / Did they all go to Paris?
-Bailamos / We dance, we are dancing.
-¿Bailamos? / Shall we dance? 

None of the sentences in Spanish change, and we know they are in the affirmative because of the absence of the mark (¿). When reading there is no way we can tell whether the sentence is in the interrogative… except when we see the upside down question mark. It simplifies things a lot. In English we see do, will, did at the beginning and we know those are questions.

The upside exclamation mark (¡) is just a copy of the above. We should never use more than one, writing: ¡¡¡Fuera de esta casa!!! but ¡Fuera de esta casa!