What is it used for?

Sawtooth Hangers are used to hang the frame on the wall.

How do I use the sawtooth hanger?

Sawtooth Hangers are centered across the top of the frame.  You either "push" (with hammer) or nail in the sawtooth hanger.  The sawtooth hanger allows you to hang small frames without the use of wire.

When should I use the sawtooth hanger instead of the screw eyes or the mirror hangers?

Sawtooth Hangers are meant to hold small lightweight frames.   Sawtooth hangers hold the entire weight of the frame in the center 1" to 1-1/2" space across the top of the frame.  If the frame is too heavy to balance on your pinky finger, use screw eyes or mirror hangers.