Well, standard conditions allows for standard reduction potential values to be tabulated, and in turn, allows us to calculate predicted voltages of different combinations and whether there is likely to be a spontaneous reaction, i.e. as indicated if the overall voltage is positive, in the same way of asking why these standard conditions are set, you could ask why, for example, the mole has been defined a certain way-these standards just serves as a reference point, and as EMF varies according to these factors-temperature, concentration, air pressure-it is logical to establish standard conditions as a reference point. How changing the standard conditions could affect experimental EMF could depend on just what you are changing, for example, if you are increasing the concentration of the electrolyte, it is likely that conductance can be faster, so you could have a higher voltage than the theoretical, only EMF measured under the standard conditions can be called standard EMF values-hope this helps
