I use it sporadically and I learned what to do pretty much by watching what Angie does on her various boards. If you peek at hers you will see that she has some that track her favourite looks from the runways, some that track various trends, some that track items she is hoping to add to her own wardrobe, one that documents her outfits, and so on. In other words, she categorizes her pins according to a system that helps her make sense of them.
Well, I don't have boards in all the same categories, but I have categorized them in a way that makes some sense to me for my purposes. I have "fashion inspiration" (so those would be looks I like for some reason...they might not be perfect but have something about them that I find inspiring -- colour, proportion, cut, etc.)..."items" (things I want or have already purchased)....a board of my actual outfits....a board of YLF friends' outfits that i really like...etc. And I "follow" people here on YLF (you can subscribe to their boards) and pin from their "finds." (Because I never find anything on my own, quite honestly.)
Another way to use it is for specific items you find difficult to style or that could wind up as orphans in your closet if you didn't search for inspiration. Sveta has some boards for "blush trousers" (I think that's what she called it) and maybe "slouchy trousers." When you are getting used to a new look, you can find out how other people have styled the item.
Over a period of about 6 months, I discovered some patterns in what I tended to pin the the inspiration board. This is how I figured out my style descriptor or rather, why it rang true for me when I finally heard it from Angie. So I would say that even though I feel like a bit of a klutz with Pinterest, it has been helpful to me.