When to parry and when to avoid?

So I'm not exactly sure when to parry or avoid. So far I've gotten through the nightclub with almost no parrying, but this doesn't seem right.

I know parrying deals structure damage to the enemy, but what's the point of avoid if you've mastered parrying.

Is it better to parry specific types of moves or enemies? What is the key to knowing which defensive options to use? Or is one just useless.