In poetry why are some syllables accented and others not?

Question:

In poetry why are some syllables accented and others not?

the same syllable which is accented in one line isnt in another.

Best Answer - Chosen by Asker:

Languages use accents to emphasize pronunciation of letters in the words

for example in spanish there are two types of o's, one with a accent and one without

I dont think it has anything to do with poetry.