After a decade of modding for Valve and Epic Games and playing mods for over a decade (Counter-Strike, Kingdoms Collide, Age of Chivalry), I finally decided to make my own video game. As an indie developer, I knew that adding the ability to mod my game was super important. What I could create in an entire year, a person or people who's passion and skills far exceed my own, could blow up my little world in a single day or month. You can make money by making a game or games. But you can make millions if you share it...
By revealing your source code, other developers may buy your game to look at the code. They may even make a mod or suggest simple code changes.
Playing games to making games is a giant leap. Most don't even know where to begin. If they see your source code, they can have an idea of where to start.
A lot of people laugh at first games, mobile games. But after a year, two years, three years... The n00biE will move up in the world. Imagine if they kept modding your game the whole time.
Any computer / cell / console that has your game installed, you could glance at the code anytime, anywhere.
4. Specific Developers
Using a framework or game engine, others who follow those may be more interested in your game.
If they've already been viewing the code, a lot less training is needed and they won't bug you to know where to go for certain code.
Did I mention mods can make you millions?
Add your source code to not only the download of the game, but inside the game itself.