This was a super-long time ago, but a kernel driver was needed for video capture back in the 90's, to control devices across I2C, etc. At the time I had a Matrox video card which did analog capture, decoding and TV-in, and so wrote the first few iterations of a kernel driver[1] to get it to work. It since got adopted by a great maintainer. Nowadays I'd guess nobody is using it because the hardware is ancient.
1: http://www.cs.brandeis.edu/~eddie/mga4linux/