I haven;t used them yet in Skyrim mods, but I don't think much changed from Oblivion. Aggression interacts with disposition, and if A>D then the character will attack. Confidence determines when low health will make the character stop attacking, and flee.
In most cases, unless the desired outcome is that the affected Actor gets killed, you want to use ModActorValue, so that the game engine does most of the work. Mod +5 followed by Mod -5 will revert to the original value, even if it was at the maximum. Ditto for dropping below zero in the other direction. If you use Set, you don't know if you're increasing or decreasing unless you get the original value first. If you're essentially forcing an attack, then Set with the maximum values may be appropriate, as a dead actor doesn't need resetting!
Where the Set form is useful, is when creating a new actor, (e.g. a summoned creature). It can avoid the need for a large number of forms for multiple levels of summoned creature.