What's the point of using, for example for weapons, fire curse or poison over increased damage?

It seems pointless putting an enchantment on that does extra fire or poison damage only a % of the time when you can the same or more normal/magic damage all the time. Even if it's to do with resistance, the % chance makes those seem redundant.

Retribution Aura seems useless, as well as damage aura on jewelry when you have Static Charge Aura. Are these enchantments as useless as they look?