Good thought, Marco.
But most. if not all variable VR's have a fixed or lesser
VA output. IF you lower the voltage, usually the available
amperage goes down as well...
I use two in my bench supply, one with a 7815 and one
with a LM338. They work well to provide a variable voltae
but the available current is not 5A (say) for the LM338
unless the voltage is quite high. At very low voltages,
the available current is 0.5A or less.
But it is a good thought. Possibly it could be rigged
as a linear supply with 2 LM338's and a LM324 regulator...
It would be cheaper than buying separate fixed supplies.
Also cheap are variable shunt and digital supplies, but
they usually have a big RF noise problem and require
some hefty filtering..