High temperature hook up wire is used to connect the heating elements from where they exit thru the insulation to the switch or controller. This wire will handle up to 850 F intermittently and 500 F continuously. This is 12 guage stranded wire suitable for loads of up to 20 amps.
This wire is not intended for use inside the furnace, but rather on the exterior where the heating elements emerge from the inside. Normal wire won't work in this area because the body temperature can still be quite high and will melt the insulation of typical wire.