I didn't care about career development or resume padding. I was just a teen! I wanted to understand how that game worked, maybe add some cool stuff to it.
Things like pointers, memory management... I learned by copying existing code. I understood it because I had experienced it. If it was code for a fireball, I had experienced that fireball, in-game.
Now I'm a full-time programmer making more money than anyone in my family ever dreamed and it all started with that open-source game engine :)