Rails::Railtie is the core of the Rails framework and provides
several hooks to extend Rails and/or modify the initialization process.
Every major component of Rails (Action Mailer, Action Controller, Active Record, etc.) implements a railtie. Each of them is responsible for their own initialization. This makes Rails itself absent of any component hooks, allowing other components to be used in place of any of the Rails defaults.
Developing a Rails extension does not require implementing a railtie, but if you need to interact with the Rails framework during or after boot, then a railtie is needed.
For example, an extension doing any of the following would need a railtie:
configuring a Rails framework for the application, like set