What if the low-level specification for your device driver demands that, between the execution of func_a() and func_b(), there should be a 50-millisecond delay? Furthermore, depending on your circumstances, the delay should work when you're running in either process or interrupt contexts. What if, in another part of the driver, you require a monitoring function of some sort to be executed asynchronously and periodically (say, every second)? Or do you need to have a thread (or several threads) silently performing work in the background but within the kernel?
These are very common requirements in all kinds of software, including our corner of the universe – Linux kernel module (and driver) ...
