It's called an ACATS (Automated Customer Account Transfer Service) transfer, and that does seem to be the primary reason it was developed.Fidelity's Money Transfer Lockdown is special-purpose. You know how, if you want to open a brokerage account at (say) Schwab using assets transferred from Fidelity, you only have to contact Schwab and then Schwab takes care of everything? Apparently there are crooks who can fool Schwab (or any brokerage firm) that they are you, then once the new account has the assets they withdraw the funds. So the lockdown communicates that you did not authorize any other brokerages to take assets from your Fidelity account.
There are other types of transfers that are also blocked.