Difference between Promote Project and Replicate media

What is the main difference between transferring of documents through Promote Project (By Custodian and Media) as compared to Backup/Restore (Replicate Media), beside the Promoted_Source_FileID, Promote_Project_ID, etc? When should I use which exactly?

Replicate media only works if you plan to ingest data in one project and then replicate them to new project. Replication has to be done in same order at which ingestion was done and you'd need to ensure that all media(s) are replicated. 

This was built for the use case where you'd have two different projects for same case. In project (say p1), you'd ingest and another project (say P2), you would replicate the ingested media in P1 and review.

However, promote does not have this restriction, you can replicate any custodian/media  or its subset and in any order. It is also not necessary that you have to promote all media(s).