The bitcoin blockchain is a timestamp server. That means it can proof that certain documents were added to the blockchain at exactly that time and not modified later, which led to a variety of applications within the BSV space. From certifying documents against the blockchain to later proof no modification of it occurred to social media posts.
While miners are not required to keep a copy of your blockchain post for all eternity on their mining servers, a whole ecosystem spawned over the years that uses the blockchain as decentralized storage system.