Flash messages are perfect for those one-shot messages that apply directly after an action has taken place inside your application.
Maybe a User got their username or password wrong and you want to show them a friendly little message. Or maybe your User just changed their password and you want to let them know everything went just swell.
Of course, you don't want to show these messages for every single page load. Just the page load directly after the interesting action took place.
Handling this by hand would be yet another chore for you to implement and subsequently maintain in your application code.
These are the sorts of things that the Symfony Flashbag was designed for.
However, there are other interesting uses of the Symfony Flashbag, and that's what this short series is all about.