Period would be the default, so no reason to include it as well. Questions and exclamations have a different tone than simple statements, so it’s more useful. It’s particularly useful in Spanish, as the word order/conjugation is changed less frequently than in English, so it’s going to help a reader understand intent more quickly. It seems like it became established as “proper” in 1754 by Spain’s Royal Academy. It also would have been very easy for printers, as they would just chuck their existing type into the tray upside down.