Why do Flask Extensions exist?
The extensions exist to extend the functionality of Flask, and reduce the amount of code you need to write for common usage patterns, like integrating your application with SQLAlchemy in the case of flask-sqlalchemy, or login handling with flask-login. Basically just clean, reusable ways to do common things with a web application. 
But I see your point with flask-sqlalchemy, its not really that much of a code saver to use it, but it does give you the scoped-session automatically, which you need in a web environment with SQLAlchemy. 
Other extensions like flask-login really do save you a lot of boilerplate code.
Lets take SQLAlchemy as an example.
Why should I use the Flask SQLAlchemy extension instead of the normal SQLAlchemy module?
What is the difference between those two?
Isn't is perfectly possible to just use the normal module in your Flask app?