Apps keep piles of files such as images, backups and logs, and a server disk is a poor and costly place for them. Object storage like S3 stores any amount of files cheaply and durably, and serves them over HTTP, so it suits user uploads or a static website. It is not a disk: you replace whole objects, so a database or anything that edits bytes in place needs block storage. Presigned URLs and storage classes are common interview topics.