To be pedantic, it is fa.mi.li.ā.rēs (five syllables, not four)
It is important because if the second a were short, the stress would fall on li and we'd have fa.mi.lí.a.rēs (a word that does not exist), and not fa.mí.lia.rēs (a word that is impossible in latin).
See for example fī.lí.o.la - little daughter - which could never be fí.lio.la.
As a side note: Italian has inherited much of its the stress pattern from Latin, but treating it in a different way:
- Latin Ī.tá.li.a (stress on the penultimate ta - with short a - because i in li is short)
- Italian I.tà.lia (stress on the last but one, as most words do)