Olá, @herweijer.matthijs The accent is a visual indicator of the stressed syllable and its respective pronunciation, and it allows us to distinguish amá-lo from ama-lo, which looks entirely the same (minus the accent), but is completely different:
amá-lo → Derives from the infinitive form amar (amar + o = amá-lo). Stress on the second syllable (-má-).
ama-lo → Derives from the second-person singular form amas (tu amas + o = tu ama-lo). Stress on the first syllable (a-).
In short, the accent is added because this is the infinitive, and without it, we’d be looking at a different word in a conjugated tense.
Olá Joseph, so if I am correct the accent is added purely to distinguish the infinitive. I can see how -ar verbs are then stressed but would you happen to know how -er and -ir verbs are stressed? Are these stressed in a similar manner and are therefore these examples?
@herweijer.matthijs, infinitives of -er and -ir verbs are also stressed on the last syllable. But -er forms are accented with a circumflex accent, and -ir forms are not accented at all. So, your examples should be written as dizê-lo and assisti-lo.