Using own css (Edit CSS or Editor ?)

First of all: I’m new to WP, so please apologize if I ask really stupid questions.

I’ve created a child theme and I wanted to change the new style.css.
Therefore I added some lines using the Editor.
It didn’t work at all. Changes never appeared.

Later I added the exact same lines using the Edit CSS and it did work.

Can anyone please explain me the difference? I prefer working with my style.css file and I don’t see any purpose in using the Edit CSS.