Initializing Transactional Replication From Backup There is an option when setting up a publication for replication to Allow Initialization From Backup. We have been creating replicated databases for several years now and have always initialized from a backkup but have never set this flag (we just noticed it for the first time a couple of days ago). The replication certainly has worked without issue all this time. 

I've found a lot of hits explaining that this needs to be used but none that explain why.

Does anyone have an understanding of what this actually does? From my vantage, it doesn't seem to be necessary but I'm thinking I have to be missing something.
Putting the comments into an answer so the question can be marked answered:

@crummel4 says, "See the third section from the bottom of http://replicationanswers.com/NoSyncOn2005.asp titled How "initialize with backup" Works, and How to avoid Pitfalls"

@Kin says, "For step by step instructions, please refer to http://www.mssqltips.com/sqlservertip/2386/initialize-sql-server-replication-using-a-database-backup/ or http://blogs.msdn.com/b/repltalk/archive/2010/03/16/deep-dive-on-initialize-from-backup-for-transactional-replication.aspx