for giving the heat to rod and make joint easily. if you are referring to Tig welding or GTAW as you call it in America the amperage depends on the thickness of metal the thicker the metal the higher...
A couple primary reasons. 1 the current reversing itself helps to break up oxides on the surface of the metal. 2 Alternating current produces more heat in the arc which is useful in welding aluminum...
Most GTAW setups use Argon. Helium was first used because it was developed primarily to weld aluminum and Helium being a lighter gas allows more heat transfer to the metal. But it is more expensive...
TIG's heat is generated by electricity, or in laymen's terms an arc.
Gas welding's heat is generated by a fuel gas and an oxidizing gas, or in laymen's terms a flame.