Thanks for your answers! I'm going to go through your suggestions in detail to see if I can start learning a little about all of this.
I should have clarified in my first message, but I was in the middle of a night shift and didn't have a lot of time:
The reason for using this way instead of using the keys on the keyboard, is because I needed to set the brightness to 0.7953. That adjusts the display brightness to 120lm/m2 which is what I calibrated it to. For certain work, I need it to be at that setting, which is impossible to match using the keys or any of the other tools that I've seen around.
@deanishe I realize I have to first understand the difference between a shell command and a normal command, if there even is one.
@politicus (as your 2nd link suggests) I actually had homebrew installed with the brightness package. That's how I was making it work from Terminal.
As I figured it seems like it was embarrassingly simple to solve, but like I said my knowledge is basically zero.
If you have any suggestions as where to start learning/reading (is this coding?), I'd be glad to take any advice.