Why is Hess' law useful to calculate enthalpies?
Consider the reaction for the hydration of anhydrous copper (II) sulfate:
This is an example of a reaction for which enthalpy change cannot be calculated directly. The reason for this is that water would have to perform two functions - as a hydrating agent and as a temperature gauge - at the same time and in the same water sample; this is not doable.
We can, however, measure the enthalpy changes for the solvation of anhydrous copper (II) sulfate and that of hydrated copper (II) sulfate and, thanks to Hess' Law, we are able to use this data to calculate the enthalpy change of our original hydration.
Using data from two reactions rather than one does double the uncertainty, and calorimetry often carries gross inefficiencies - especially in the school laboratory; however, this method is our only option given that we would not otherwise be able to obtain our desired data.