When is it better to have a high voltage and when is it better to have a high current?
You want a high voltage in e.g. electrical power transmission. The transmission losses are \$I^2 R\$.  So, for the same power (e.g. 10kW) it is better to have 10kV and 1A than 100V and 100A. 

For welding you want to have high current and low voltage, because the current determines the energy in the arc.