Oh come on, worst case scenario the game releases on the release date. Costs nothing.
Alternately, some people get some exposure to games they might not have played and, as we can see already, the game comes out sooner.
This is voluntary and works to promote indy games, wow, how terrible, a company with a large amount of marketing power supporting indy titles rather than, say for example, making people play AAA listed or orange box games to promote an earlier unlock.