I am polling a value on a device. I am not so interested on the actual value returned by the device on the poll. What matters the most for me is the variation of that value over time or simply compared to the last polled value.
For example, I could be polling a device that would tell me how many liters is left on a generator fuel tank every minutes. Of course I can compare the polled value to a threshold at which I want to be alerted (like low level fuel). However, If that tank has a leakage or if the fuel gets stolen, I want to be alerted on the variation of fuel level, before it gets to the low fuel threshold.