So given that these are made as humbuckers why would anyone want the
option
to pick up more stray EMI and also get reduced output?


Because they have a different sound.
You may want a low output clean sound that does not distort the amplifier,
and also be able to switch in a different sounding high output option that
will overdrive the amplifier for solos perhaps.
In effect the switch makes one pickup into two.


Gareth.