It gives us an idea that which element is more reactive , or simply it is very useful in case of the study of displacement reaction. that which metal can displace other metal from the solution.
K > Na > Ca >Mg > Al > Zn > Fe > Pb > H > Cu > Hg > Ag > Au
From this series we can easily find which one is more reactive and which metal can displace the other metal in a reaction.
Lets take an example
Zn + CuSO4 → ZnSO4 + Cu
Now see in the series that Zn ( zinc) is more reactive than Cu (copper).