This is used to make a chemical take back electrons when it rather wouldn't.
Cu + Cl2 --> 2CuCl2 This is spontaneous. It is a REDOX.
Cu --> Cu2+ + 2e- (LEO-oxidation)
Cl2 + 2e --> 2Cl- (GER- reduction)
CuCl2 --> Cu + Cl2 is not spontaneous but can be achieved via electrolysis.