Don't know if this is true for all software houses, but my employer writes software for networking hardware. We have a major/minor/patch heirarchy, and there is a systematic way of how they approach stuff.
A major version introduces substantially new features. (1.x)
A minor version adds new functionality onto existing features (X.1)
A patch can be an optimisation patch, a feature patch, a bug patch or a security patch.
However, it usually goes that the first 6 months after release, there's a lot of new functionality, optimisations, and bugs. After that, it's all bugs and security fixes.
It would be different for Nvidia, but not substantially so. First year after the release of the 30X0 generation, there would have been a lot of new features and optimisation for new games etc, based on collected real world data. After that, it would be mainly bug fixes and security patches, because the bulk of the team would be reassigned to working on the 40X0 project.
Since vulnerabilities in video drivers would be impractical to exploit for criminals, after the first year, I don't really bother to update drivers unless there is something I read that will help. It won't improve performance, and it's unlikely to introduce a new featuee that is supported on my Hardware.