Simple: you can program the Arduino without needing a GUI.
One good example: You have an Arduino connected to a Raspberry Pi built into a robot. You SSH into the Pi, edit the Arduino's program, and upload the new code. All from wherever you happen to be.
A second example: You can integrate Arduino compilation into your own software. Such as a web interface for compiling your code. I know of one such project where the creator has made a custom website for editing their robot code and using UECIDE's CLI (which pre-dates Arduino's) to compile and upload the code to the target board.
And yes, you have an outdated context of Arduino development, as evidenced by your use of the phrase "fooling around with". Arduino is not a toy. It's a development platform and educational tool. Maybe back when you were "fooling around" with it, it was little more than a toy, but things have moved on a long long way since then. The Arduino ecosystem boasts boards with chips from every manufacturer you can imagine to fill every niche you can conceive. The API and available libraries have certainly matured.