After a customer login, he or she will have access from same device and browser to some its data even after session expired. For sake of security the sensitive areas like account, checkout or password change require unauthenticated customer to login again.
Unauthenticated customer granted access for interacting with a store: add products to basket and wishlist, viewing products with customer's group discounts as well as tax & delivery calculations with default customers address.
Unauthenticated access minimizes the customer's effort by preventing to enter login details for each time.
This access will be possible until user logout.