Customers should not be confused with users. They are fundamentally different. Users are, as you'd expect, people who can log into your storefront and make purchases. Customers are records that hold information about either a customer or company that a user can be associated too.
In most cases, you will find that your customer records will only have one user attached to them, but you have the flexibility to attach multiple users if your store requires it.
A use case for this might be that you have a customer record that represents a company and you want multiple users to be associated with the customer. This would allow them to share information without duplicating data across multiple users.