The unusual thing about Pinterest was that even when it was an exclusive website, it still had thousands of people requesting to join up. I guess that it is probably because of it's modern user interface and friendly community.
The reason you would upload a picture to the app is extremely confusing, you don't add filters or effects you simply upload it to a board and then other people re-pin it. It does take some time to get your head around the board idea. I suppose that they are basically just like albums so you upload your pictures to different boards so you can categorise them or find them easier.
I honestly wouldn't recommend Pinterest, I only joined it because everyone else was using it and now I've signed up, I don't actually use it that much. It is easier using Flickr or Instagram.