Here's the basic principle. Everything wants to go to ground. If you touch a ground wire to any line, the signal will immediately go to ground. (That's how most kill switches work, by the way.) A shielded wire is really two wires. The wire on the inside carries a signal. The shielding acts as a second wire, and it connects to the ground. That's how shielding works. If any stray interference is in the vicinity of the wire, it will connect with the shielding and get sent immediately to ground, before it ever has a chance to interfere with the signal line.
A pickup selector switch deals only with the signal line. It's not typically shielded or grounded, though some might argue that shielding would be helpful.
All grounds must connect to the ground on the output jack. Otherwise, they're not grounded. The ground is carried through the output jack, through the cable, through the amp/pedals/whatever, then sent to the electrical ground in the plug.
I'm not sure I entirely understand the question, but I hope that helps.