The only thing I can come up with is timing belt / chain flex which can cause a very small amount of timing difference. I guess they could detect if the belt was put on wrong, skipped a tooth, stretched, or broke. Everything but stretched you would notice the problem and timing belts don't stretch much last I heard.
Other than those failure states, you should be able to program everything using just a cam position sensor. So why do cars have a crank sensor in addition to a cam sensor? This seems redundant and unnecessary. Or is its job just to tell you something major is now wrong with your car after the belt snaps and valve mashing commences?
Here is an example of a Honda's parts list as you can see it has both sensors.