A Deferred Save means the page that you Obj-Save'd is not immediately sent to the database as either an insert or update, then committed. 

This allows the transaction to wait to see if a downstream error is encountered later when attempting an action such as a flow action submit. If so, the deferred save list does not need to be processed, instead it can be removed. Think of this as a "rollback" that occurs in Pega, which is middleware within a three-tier architecture, as opposed to requiring a database rollback as needed within a fat-client, two-tier, client-server architecture.

If the deferred save list did not exist, a compensating database action would be required for every committed insert or update operation. It is too late to rollback a transaction after it is committed.

This is why you always want avoid using Obj-Save + Write Now for complex objects such as cases. For simplistic Data- objects such a Data-WorkAttach-File, and depending what is being performed, such as uploading a file as an attachment, it is acceptable to use Write Now (see Work- AttachFile)