One use is to determine the electric potential need to maintain a given ion concentration across a membrane.  
One use is to determine the electric potential need to maintain a given ion concentration across a membrane.  Some pH meters use it to determine the concentration of hydrogen ions by measuring the voltage on an electrode.  However, it is most often used in the other direction, when ion concentrations are known, and electrode potential is not.  FTP, identify this equation, namesake of a German physical chemist, which predicts cell voltage under non-standard conditions of temperature and concentration.
One use is to determine the electric potential need to maintain a given ion concentration across a membrane.  
Nernst equation