Many reactions contain several steps. If so, then each step will have its own enthalpy change. If all of the changes are added together, then the sum of that addition will be the net change for the system. In other words, ultimately, the only thing that matters is what the net result of a reaction is. Hess's Law is frequently stated as Hess's Law says that the net change in enthalpy of a reaction will be the sum of the enthalpy changes that occur along the way.
Many reactions contain several steps.
If so, then each step will have its own enthalpy change. If all of the changes are added together, then the sum of that addition will be the net change for the system. In other words, ultimately, the only thing that matters is what the net result of a reaction is. In such a system, a reason exists for the process to proceed on to completion. Some of these process will require a specific temperature in order for the process to occur.