I understand the construction of these targets but not the reason for gluing them. Can some one educate me?

They aren't glued.  They are compressed. Then They are sprayed on the exterior with like a rhino liner..

You don't have to glue them just compress them, you can do the same thing with cardboard. I would thing glueing them would make a mess, he arrow generats heat as it sticks into the target. They make a great target.