It is important to know when and how elements and compounds change state in order to use them.
You can't drink solid or vapor water. You need it in a liquid state. Similarly, other compounds are more useful in a particular state. The important part of state changes is the amount of energy that must be added or taken out to change the state.
The temperature of a phase change remains constant while the energy is exchanged. Only when all of the compound is in a particular state will a change in energy input change the temperature of the compound.
This is the basis of both the steam engine and air conditioning systems.