Why is there a ground wire on turntable?

In respect to this, why do turntables have a ground wire?

It (a separate ground wire) is a method to help reduce ground loops and unwanted common mode currents in the audio signal path, phono cartridge to phono stage input. Remember that a signal does not just stop when it comes to a ground connection.

Beside above, where does ground wire go on turntable? In general, a single turntable ground wire should be attached to the ground post or a chassis screw and the turntable on the back side of the amplifier or the receiver as well.

Just so, is it necessary to ground a turntable?

Turntables that do not have a built-in preamp need grounding. Turntables that need grounding feature a cable that needs to be attached to your preamp (whether it's an external preamp, mixer, amplifier). Turntables that are not grounded will produce an obvious humming sound.

How do you extend the ground wire on a turntable?

You could use any electrically conductive wire, preferably one with an insulating outer sheath (even speaker wire). You'd strip both ends of the extension wire, connect one end (just bare wire) to the small spade on the ground cable, and connect the other end (just bare wire) to the grounding pin on the receiver.