Açucar without an accent would be pronounced as all verbs ending in "ar": açuCAR.
Verbs: coziNHAR, muDAR, estuDAR.
The "R" at the end of the word makes the last syllable strong, besides some other consonants which also do that, and the vowels "I" and "U".
Last syllables ending in vowels "a, e, o" alone are weak.
So, you would be right if it didn't end with "R": caMIsa, meNIno, Usa, FRUta, CAsa, PASso. But in none of these cases there would be an accent, it's already stressed.
- Compare "(ele) Usa" with "usAR".
- Or "(ele) CANta" with "canTAR".
Accents are not randomly used, they have the role of stressing the silabes that would be weak without them.
- Without accents: PAguem, VAlem, LAMbem
- With accents: alGUÉM, aLÉM, tamBÉM.
Tilds also stress the silabes, but that's not it's main function. Accents prevail over tilds when both exist in the same word.
I'm no portuguese expert, but in my experiences with equivalent spanish words, the word "azúcar" is accented on the second to last syllable because, in spanish anyway, if the letter "r", "l", or certain others end the word, the accent is automatically relocated to the last syllable. Assuming the same for portuguese, that is the reasoning.