nVidia's own control panel is not always very intuitive to use. Especially the 3D rendering settings are very cryptic and difficult to navigate. Even the new control panel introduced with the 9x.xx line of Forceware drivers isn't a real improvements for these problems.
nHancer is a tool to improve the usage of these features. Both normal and advanced users should find nHancer to be a big step forward.
Additionally, nVidia drivers include a unique feature, that allows them to define profiles for every game, which control various image quality, compatibility and optimization aspects. But in the standard control panel, this feature is even more cryptic to use, so most people never bothered about it. Learn more and download here