In Portuguese, they indicate that a syllable is stressed and alternate ways to say the vowels. e.g. "país" is stressed in "i" and means "country", while "pais" is stressed in "a" and means "parents". Tilde (~) indicates that the vowel is nasal, e.g. the "ã" in "São Paulo" means that it sounds like the "u" in "sun"; the default sound of "a" in Portuguese is the same as in "car".
because you know the stress syllable by looking at the word. take Desert and Dessert, do we say DES-ert or des-ERT. Also in portuguese, at least, I can know which "e" sound [1] each "e" in the word makes by knowing this (well, almost, but not completely, but much better than English.)