Here is a WAG. It allows you to switch from big power tubes, i.e. 6550's/KT88's etc to smaller power tubes i.e. EL34's/6L6/5881/KT77, etc. The amp probably has two different settings internally, one for each class of tubes, yet within each range employ it's auto bias circuit.
BTW, why don't you just call the USA distributor and ask?
BTW, why don't you just call the USA distributor and ask?