New to HP prime

[std]

Hello again friends.
It's really nice to see that there is an active community with friendly people helping each other. It may sound like a stereotype, but it's not the case in every forum.
I am back again with a question. When creating a new program, when should I check the CAS checkbox in the starting dialog? I am going to write a program using CAS variables and functions, if that helps.
A youtube tutorial that I watched, where CAS was not checked, although it works with expressions containing X variable.
https://youtu.be/A7PQ366FF-g?t=9m

The documentation is so outdated and not of much help. I read somewhere that an up-to-date version is coming, but who knows when...