I noticed that too, and had a look around. No other language uses the accent marks like in Portuguese.
The accents in Portuguese serve two functions, to show the tonic accent and then to show if the vowel is open or closed. French only shows the later, and Italian, in theory shows both on the final syllable only, but they tend not to be rigorous. The first Italian method I had in high school actually only used grave accents. Is the modern accent scheme in Catalan the workings of Pompeu Fabra? I've always wished Italian would adopt the Catalan rules of accentuation, but I suppose it poses a problem nowadays when vowels can be open in one region and closed in another.
In French circumflex accents show vowels that became long when the following consonant was dropped. Later on, the vowels shortened again, so nowadays ê and è sound the same. Could the Portuguese ê, â, ô have originally been long too? Actually maybe they still are. My Portuguese pronunciation isn't the best.
Regarding Portuguese. If I'm not mistaken the acute accent is much more frequent than the circumflex, and shows word stress in a similar manner to Spanish. My theory is it's just more convenient to use an acute accent when a couple words in every sentence need an accent mark. This was set up in the days when manuscripts were written by hand. It's quicker. Also showing stressed words is also more important in a language like Portuguese than vowel quality. All natives know the quality of the vowel anyway. The circumflex adds the extra function of closed vowel quality and could slowly have been added and marked when absolutely needed. Do very old Portuguese texts have circumflex accents?
I wonder if â in a word like Alcântara is meant to be pronounced closed.