Okay, call me dense or blind, but I see that several Avatars have a favored action, and if not the favored action is charge.
But for the life of me I cannot find anything about what a favored action does in game terms!

The favoured actions are used, for example, when under hatred effects. If the Mind check is failed, the Avatar is forced to perform his favoured action against the hated enemy.
So, for example, a Barabrian hero would charge, but a spellcaster would cast his favourite spell against the hated Avatar.

As new Avatars, ... join the party they will be used in the future in more situations.

Cool, I must have missed it!
Thanks G.