As to why you'd use Pinterest, its main purpose is as a place to find and store references. This can be for practical or purely inspirational purposes.
For instance, I pin images of pipes and pipe-related photos I like to that board. I don't really consult it when looking for new pipes or new tobacco, but I like looking at it from time to time to remind myself of what's out there. I also browse other people's pins for the same reason (and here I might definitely get inspired to look for something specific to buy).
On the other hand, I have a woodworking board where I pin ideas to use on my own work, and as references of techniques that I want to learn.
And so on. My wife is a designer and uses it regularly. It took me a long time to figure out what the point was, but now that I do it can be handy.