Comment by jbay808

It's certainly one of the reasons. (There are also others, like building a good reputation to help attract and retain talent).

For example, in 2009, in the world of physics engines, Havok (acquired by Intel) and PhysX (owned by Nvidia) were pretty much the two notable market leaders. Physics acceleration was becoming a big deal in games and an important complement to GPUs, and AMD/ATI found itself without a horse in the race. So did it develop its own proprietary physics engine? No, it threw its weight behind the open source Bullet physics project, to "commoditize their complements". They don't need to make money from engine licensing; they need to present a viable alternative to PhysX to keep Nvidia from getting ahead. For that, the lower the barrier to entry (free) the better, and the lower the barrier to contributions (libre) the better.

Now we're seeing Facebook (big stake in VR) contributing to Blender (3D content creation, and so a complement to VR). And so on!