Chad Callihan explains why we need tail log backups:
When you are migrating a database from one server to another, how can you be sure to backup all transactions? Sure, you can notify the client and let them know “there will be a short outage at 8AM so please stay out of the application at that time.” Can you really trust that? Of course not. Let’s demonstrate the steps needed to include all transactions with the tail-log backup.
Protip: if you build your application such that nobody wants to use it, you can migrate the database much more easily. Assuming you don’t want to follow that outstanding advice, Chad has you covered.