Collections is important For organization and provides a way to animate multiple Shots in your Scenes a little easier. You can also use instances and Collections to save modeling time, and have faster renders.
 

Beginner

This is a Video that explains Collections.  One Tip was to be aware that whatever instance your working in, and the original is hidden, changes made will show up on the original and you might have to move the change to the collection of the instance.  Unhide the original to see your change. Another was how he uses Collections and instances to mirror parts on an object, very useful. Then he talks about how using instancing reduces your gpu rendering times.

Intermediate

Here He talks about Collections and Instances. 

I've posted this video on the site in several places, there's so much good info in it.  He has a trash collection (he could of named that something better, like Originals or Base) And had all the objects in his scene in that collection. Then he had Collections names shot1, shot2, etc, and each Shot has all the objects for the shot in the it. The Trash collection I think he was using to quickly populate his shots with out having using the append feature to import objects.  Speeds up workflow.  This was a great tip that he doesn't talk about in his video. 

Advanced

This is one of the better uses of collections and using instances I've read here.  He makes instances of an object and changes the pivot point so he can easily place objects in different situations. Then he names the collection a meaningful name to help him place objects on the floor (face up or face down) or on a hanging pole.