On tutorials, have you gone through the section in the Interactive Tutorial itself? There is a whole section devoted to using them, near the end of Part II.
Of course that mostly just goes over which buttons to click and what happens when you do so, rather than explaining how you can use the feature. That is more difficult to do—as with most things in Scrivener, the how is largely up to you, and given how flexible its features are in general, they can do quite a bit. One example is to save a Binder selection. Say you have ten documents selected from all over the Binder, so that you can see them together in the main editor view (using Corkboard, Outliner or Scrivenings). Now say you want to come back to that session later, but it might be a while before you do so. You could just go on working, and later manually reselect all documents—hopefully remembering where they all are—but it would be easier to just use the Documents/Add to Collection/New Collection… menu command (or click the + icon above the Binder tabs if you have them shown). When you are ready to work with this selection of text documents again, just click the tab, select all of the items (Ctrl-A) and either work with them right there, or use View/Reveal in Binder to return to the Binder view with these items selected and fully revealed (in case there were in collapsed folders).
A more dynamic usage of the feature might involve using it as a to do list. You may note that in new projects there is a built-in “To Do” status that you can apply to documents using the Inspector. You can use Project Search to look for status alone, and type in “To Do” to find every document in your project that you’ve flagged as needing work. That search can then be saved as a search collection for future use, via the same magnifying glass menu in the Project Search tool that you would have used to change the search type to “Status”. As you complete tasks, you can mark things as “Done” and when you visit that collection tab in the future they will no longer be listed.
Collections can be a workspace tool as well. To take one real-world example that I’m using them for right now: I have two folders in my Binder that I’m working with, and they both contain several dozen documents. It would be unweildy to work with them in the Binder—lots of scrolling around since both of them expanded add several screen-heights of data. So instead I’ve selected both of those folders and created a Collection out of them. Now I have two entries in my list. I can click on one folder to see its contents and work with them, click on the other to virtually jump to an entirely different spot in the Binder, and so on. As I work, I sometimes pull out sub-items from these folders to “elevate” them to my top-level workspace in the Collection. It’s a way of temporarily bookmarking things I’m frequently returning to—stubs I’ve started writing but haven’t finished yet, and that sort of thing. I keep this relatively clean so it doesn’t get cluttered up with old irrelevant items.
I’ve included a few more ideas like this in the user manual, §8.4, Using Collections, and the forum has many old threads on practical applications if you do a little searching around.