Why do words like árbol and fútbol have accents when they normally wouldn't be there?

Answer 1

For words that end in "n" or "s" or a vowel (which covers most words in Spanish), the penultimate (second-to-last) syllable receives the spoken emphasis. For all other spellings, the last syllable is emphasized when speaking. For exceptions, the accent is used to indicate the stressed syllable. So without an accent, árbol or fútbol would have the stress on BOL. Since the spoken emphasis is on AR and FUT, the accent is needed. Similarly, the accent can appear on the last syllable, to indicate it is stressed even though the word ends in a vowel or "s" or "n," such as "hablaré inglés" ("I will speak English").

 

Accents can also distinguish two words that are otherwise spelled the same, frequently interrogatives. (These are usually superfluous in terms of indicating the stressed syllable, as discussed above.) For example,
"Cómo" = "how?", but "como" = "as" (or "I eat").

"Sí" = "yes," but "si" = "if."

Answer 2

They are there in order to tell you where to place the enphasis on the word