"Nel ristorante" would translate to "inside the restaurant", which would work if you were to be talking to someone who wanted to know the exact physical position of someone else that is close by. "Al ristorante" means "at the restaurant", which would be the answer to those questions which deal with both longer distances and less of an immediate need to know where that other person is (think perhaps coming home from work and upon seeing the absence of person X, you asking person Y "Where is X?" to which Y would respond "She went to the ..." so there's more of a factor of confusion.
Hope this helps.
Why is it al and not a? Based on the grammar I've received you'd only use the article if ristorante is specific, i.e. modified or identified in some way. For example al ristorante novo or al ristorante ad angolo. But if it's only ristorante unmodified, there'd be no definite article so just a ristorante.