Here's my understanding of how it works, based on the puter.js docs [1]:
If I'm developing a frontend app that could benefit from cloud storage, I can load in puter.js as my "backend". I don't need to worry about user auth because puter.js will automatically ask the user to create an account or log in. I also don't need to worry about managing & paying for cloud storage because puter.js will take care of that on a user-by-user basis - including asking the user for payment if they go over their free limits.
I haven't actually used puter.js yet. But if I understand correctly, this could be a really powerful model. As the developer of a niche app whose purpose is not to bring in revenue, puter.js seems like a very reasonable way to pass on cloud storage costs to end users, while also reducing development effort!