The idea is to give developers a better idea of how it all works, plus it means that if there's something essential developers need, they can look to add it in themselves when creating their games. Open source (and even "source available") is just far better than hidden proprietary code.
YoYo Games will continue to update it, so it's not just a code-drop and forget type of deal. They are also encouraging others to create Pull Requests to improve it.
Their own Gurpreet S. Matharoo mentioned:
As a developer, having access to the HTML5 source helps me understand how functions work behind the scenes, so I can get the most out of them.
In addition to getting to look behind the curtain, I can now submit my own changes if I happen to come across bugs or functions that I can improve.
Seems like it makes it easy for game devs to tweak and test it too, with the GameMaker Beta allowing you to pick a different runtime and select your own downloaded version of the code. So you can easily test it locally for your own games, and before potentially sending fixes directly to the new GitHub repository.
Good stuff. Hopefully this is the start of more of GameMaker being open.