Reactivity ordering of metals is important
1. We know which metal (calcium, sodium etc.) will replace the metal in another compound with which it is reacting. That is, we can predict the products of the chemical reaction.
2. Which metal has higher oxidization potential and more reactive, - this fact is important in making chemical compounds that we want , from other compounds.
3. The reactive metals may need to be protected from environment, other chemicals and gases. It is important to choose reactive or non-reactive elements depending on situations.