To get more people running heavy number-crunching algorithms on their monster graphic cards? Like CUDA stuff. AFAIK most these applications are on Linux platforms.
I think that more and more software is becoming a means to sell your hardware. I would assume that Nvidia wants to utilize as much of the community as possible to push the next wave of Linux based mobile devices. They aren't really making money off the software anyway, so I doubt it would hurt them to open-source that and it can help them to push their products on more linux based devices.
I imagine they will not open source their Linux drivers. While I wish they would, I am not certain that would make business sense for them.
They get to put nice empty PR statements as they are part of the Linux foundation. To the common man on the street it means the same as they are now fully supporting Linux.
Credibility when the time comes to woo manufacturers to build Linux-based products with their hardware?