What this is and where it shows up
Every app that remembers who you are needs this. Registration and login is the front door — the mechanism that creates an account, proves identity on return visits, and keeps one person's data separate from everyone else's.
It shows up in almost every project: a client portal, an internal tool, a booking system, a membership site. Even apps that feel simple ("it's just a form for my team") usually need accounts the moment more than one person uses it, or the moment any data is personal.
The building blocks are: a registration flow (collect details, create a record, confirm the person is real), a login flow (check credentials, start a session), session management (keep the person logged in safely), and account recovery (what happens when they forget their password). These four things are more connected than they look, and that's where most new builders come unstuck.