The button is used in a Smoke situation - hence the light on the button illuminating if Avionics Smoke is sensed. It removes the power from Gen 1 to AC bus 1, and when used as per the QRH, in further removing the power from bus 2, you go into EEC and hopefully isolate the problem if it electrical.
It removes over 70% of the aircraft electrical load.
However, as stated above, it prevents gravity feeding, as a pump in each tank stays powered, which wouldn't happen if you removed the power using the Gen 1 button.
Why?
Because the smoke will probably not be coming from a pump in a fuel tank.
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And just to clear something else up, [from the airbus crew training manual] you don't put the Gens back on before landing to get the instruments back.
It is to regain the anti skid and improve your stopping distance, if required.