A diacritical mark is a mark that declares the pronunciation of a word. We have seen until now the accent grave (`; e.g. è) and the accent aigu (´; e.g. é). Further we have seen the accent circumflex (^; ê).
Accents are diacritical with the letter e and o. They indicate the way it is spoken. An é is spoken in a closed way, the è and ê are spoken as open e. An ô indicates an open o, as an o can be spoken as open or closed o.
With other letters (a, u, i) the marks do not indicate the pronunciation, but rather the grammatical function or the meaning in case of homonyms.
One could again think and discuss the use of this, because other languages get along just fine without these marks. However, we will refrain from this and cite the philosopher:
The tradition of all the dead generations
weighs like a nightmare on the brains of the living' (Karl Marx)