Actions are like mana, health, cards remaining in spellbook, or any other resource you can think of: They are critically important when you have a shortage, and less important when you don't.
Yes, all actions should have a purpose, but this doesn't mean all actions are equally important. It is possible to be in a situation where the best possible action is no action at all. (For example: You are trying to save mana so you don't want to cast, there is no enemy in the square to attack, and you have a big creature Guarding you, so you don't want to Guard. Therefore, you flip your action marker and do nothing.)
Every action has a cost, whether it is mana, positioning, or putting yourself on Guard and making yourself a valid target for attack. If you are more limited by costs than actions, then actions are not as crucial.
Casting spells without purpose, whether it's AoEing one target or blowing incantations at the wrong time, is a waste of both actions and resources. Sometimes it's the loss of actions that really hurts you. Sometimes it's the loss of resources.