Most of it is unremarkable: bank correspondence, tax notices, supplier invoices, insurance renewals, and a good deal of advertising. A smaller part is not unremarkable at all — statutory notices, registered letters, and documents from regulators or law firms that carry a date and sometimes a deadline.
A mail-handling service exists because those two categories arrive through the same letterbox and cannot be told apart from the outside without someone looking. The point is not to store post. It is to make sure the second category is recognised on the day it arrives, and that the first category does not bury it.