just switch the + and - wires from one pickup combined with the other pickup, you get outta phase wiring. You will need a toggle switch or pushpull pot for the switching between in phase and out of phase.
geetarers do it coz they want sound variations of the instrument for whatever sound they need, for the context they created.
Some people are happy with 5 sounds, some need 10, 20 sounds, some cant live with more choices while some others love to have more, to play with.
heh, its a bit like driving. Some happy with cars to travel around and some used it to travel around cum racing. They aint weird, but just having diffferent needs than us.