Linux is important, because there are not many good engines supporting it. It`s an open source OS with open source software. Therefore usual Linux gamers don`t want to have a "Gears of War" or other common games. Many users are developers and enjoy playing indie and/or hobby games. The market for a tool like Gamestudio is huge.
The most successful and highest-rated games on Linux are "Battle of Wesnoth", "OpenTTD", "Quake Live", "Nexuiz", "Super Meat Boy" and "World of Goo". It`s a wonderful playground for indies and hobbyists.