Travis Field, fieldres on you tube did a video on the subject. Steel melts because of the heat generated by a high resistance ionized gas and the arc of electrons flowing across. Electrical power is measured in watts. Electrical pressure is measured in volts. volume is measured in amperes. Volts x amps is watts. 10 volts x 100 amps is 1000 watts. 100 volts x 10 amps is 1000 watts. The energy is equivalent. In vertical welding if we produce the wattage with lower voltage, and higher amperage, we can drag the rod, or even push it directly into the joint to burn deep pockets in the cooler workpiece. The molten metal surrounded by cooler steel freezes very quickly, preventing drips.
Conversely, higher volts, lower amps will tend to snuff out the arc if held too close. the longer arc needed will tend to melt less deeply into the workpiece. Less contact with cooler steel means slower cooling of molten filler metal.
Other situations like thin metal laid flat work well with more voltage, less amperage.
Most welders don't allow the change in voltage. Some old Lincoln engine welders do. For the rest of us a very tight arc gives less voltage, more amperage. Longer arc gives more voltage, less amperage.