Many times we assume that molar concentrations are 1M....pH is 7, and if gases are present, they are at 1 atm...and 25C The Nernst equation allows one to calculate cell voltage at DIFFERENT CONCENTRATIONS.
For example.....If you were to do dental research, and wanted to measure voltage generated by a metal touching an amalgam filling, you were need to use the Nernst equation because ions would clearly not be at 1M.
Make friends with this equation..... show it a little love.
Hope this Helps
Dr. Romano