On machines with dynamic scaling of the CPU frequency, any number you arrived at would only be valid for the duration of the instruction during which you read it.

With apologies to NationWide: It doesn't work like that! Or rather, I'm not sure that it does. But in anycase, even if it does, the ability to detect that it varies, even if not exactly when it varies, is still useful information.

For the why. Go look up the RDTSC instruction and think about how that might be utilised to provide a very high resolution elapsed timer. And then think about how that would be affected if the clock frequency is varying. And finally, how knowing that it does might be used to (at the very least) warn that the VHRT may not be accurate.

There is also the possibility that (for example) that there might be an interface to disable the variation; or that changes to the clock rate might be disabled for the duration of (say) a critical section; or ...