When to use step up and step down transformer
It is all about electrical power and energy.
An arc need a higher voltage so that the jolt of electricity can jump over(or through) air. This is why more voltage is needed than current.
And a heater needs more current than voltage. When current goes through a wire it can produce heat.  The heat power is p=I^2.R 
More the current is there,  more the conductor is hot.
That is why a step down transformer is used for situations like this to increase the current.