The idea can be to bait developers into targetting Steam for Linux. Then port this largely unmodified to Windows, using the same APIs and libs (be them OpenGL, SDL, OpenAL etc.), same scripting languages available, and even a build toolchain not too far off with MinGW and MSYS.
Afterall, 95% of what runs on Linux runs on Windows too : from sed, grep, vi, to evince, aucadity, gimp, blender, libreoffice, abiword and so on and so on. Even X11 if you wish. And the big name servers too : Apache/MySQL/PHP, nginx, PostrgresQL, another unending list.
This is a frontal assault on DirectX, which has been the big computer gaming platform those last 15 years.