What can be done with PDAs?

Identity and user data storage is the simplest. Instead of building up your own backend, you can use PDAs as your user identity and data storage.
Depending on the complexity of your application, it will fall into one of these design patterns:
Building on decentralised Personal Data Accounts is a mindset shift. You should get advice on figuring out which of the above design patterns would suit your architecture best to reap the most beneftis from using PDAs.