I've had situations where we had to add a small functionality to a driver. For instance a secondary functionality that wasn't supported in the upstream version. It's very rare for drivers of complex components to support 100% of the functionality.

An example that comes to mind was a video chip that also had a GPIO pin that we needed to control. The chip had a driver in the kernel but it didn't have support for the GPIO (it is after all a very secondary function). That required a small patch.

It would of course have been possible to upstream it but then that meant implementing a "clean" patch (using the kernel GPIO API etc... instead of a one liner to toggle the PIN high on dereset) then submitting it upstream, maybe doing a couple of back and forth... Some will bother to do it, many won't.