The STL is a collection of data containers and ways to manipulate the data we put in those containers. Or to be more specific, it is a way to store and manipulate different types of C++ variables and classes.
We can think of the different containers as customized and more advanced arrays. The STL is part of C++. It is not an optional thing that needs to be set up, such as SFML.
The STL is part of C++ because its containers and the code that manipulates them is fundamental to many types of code that many apps will need to use.
In short, the STL implements code that we and just about every C++ programmer is almost bound to need, at least at some point and probably quite regularly.
If we were to write our own code to contain ...