Intonation is different in English and Spanish.
Let's consider questions first. In English, the intonation of a declaration and a sentence are identical until you get to the last few words. At that point, the intonation rises for a question, and falls for a declaration. The question mark at the end of a sentence is sufficient to warn the person reading out loud to raise the intonation at the right time.
There is a particular form of questions in English that illustrate this intonation nicely. It's the form where a declaration has a negative question tacked on to the end of it. Example: "You went to the movies yesterday, didn't you?"
In Spanish, the intonation of a question can be different than it is for a declaration right from the beginning of the sentence. There could be a rise in intonation at the point of the main verb, with a lot of subordinate clauses to go before the end. In addition, two sentences that use excatly the same words can be a question in one case and a declaration in the other case. The only difference is the intonation.
Example: "¿Fuiste al cine ayer?" The first word of this sentence has to be intoned in the way that marks a question. The example isn't long enough to illustrate the problem, but you get the idea.
A person reading Spanish out loud needs to know that the sentence is a question right from the outset. Otherwise the intonation won't come out right, and the people listening to what is being read will not understand that it's a question. The upside down question mark meets this need. It's a need that doesn't exist in English, at least to the same degree.
The upside down exclamation mark is analogous, but I can't think of an exclamation that's long enough so that the final exclamation mark wouldn't do the job.