Originally Posted by PP Mguire
In simple drivers are just a piece of software that helps your OS communicate with the hardware better. In terms of video cards the companies tweak their drivers to help improve performance in games and fix bugs as software progresses.
Couldn't have said it better.
A recent practical example is when the game "GTA V" was released. Nvidia had to release a new driver at this point, as for some reason, using the old drivers caused the game to lag and perform badly (even for those people with GTX980s etc).