Orders need an owner.
Carts, addresses, order history, all of it belongs to a specific person. The system has to know who is asking, and prove it, before it hands over anything personal.
Two different questions are hiding here: who are you, and what are you allowed to do.
Prove it, then remember it.
Authentication proves who you are, checking a password the server never actually stores in the clear. Then sessions and tokens let the server remember you across every following click without asking again.
Now protect it.
Accounts attract abuse. Rate limiting stops password-guessing, a second factor limits stolen-password damage, roles decide who can do what, and everything personal is now data you are legally and ethically responsible for.
You worked through Phase 3: give customers accounts. 12 new concepts joined your architecture along the way.
Next problem: You cannot build a bank. Money moves safely through other people's systems.