Simply put, they are super cute bags and backpacks specially designed so that you can pay tribute to your favorite fandom or character.
In Japan, these are called Ita ("pain") Bags because love hurts and these bags are no exception. As they get loaded down with your tokens of love, they get heavy and harder to carry. Plus, all of those beautiful tokens add up and hurt your wallet.
How you decorate your bag is entirely up to you and that's where the beauty in these bags really lies. I recommend using craft foam cut down to size to hold your pins so you don't poke holes in the bag lining and you spare your hand from scraping against pin backs as you rummage around in the main pocket. But the possibilities are endless!