The 2.5” piece that is connected to the coax is half of a parallel plate capacitor. Hook up a battery there, and the plate will acquire a charge. And the opposite side, an equal but opposite charge. Apply an AC voltage to the plate, and an opposite alternating charge is induced on the opposite side. In essence, the capacitor passes an AC current thru it. The antenna itself is ‘resonant’. Ever watch two kids, wiggling a rope between them up and down? If they do it just right, they can produce a ‘node’ in the middle. That is, the rope doesn’t move at all there, while there are large up and down oscillations at their hands. In the Archer antenna, there are large voltage oscillations at the far, open end. But at the end close to the feed line, there is a node: the voltage there is zero, unchanging. Since the voltage is zero it does no harm to connect it to ground, since it’s already at zero volts, no current will flow. You could disconnect it from the ground leg if you wanted, but it conveniently provides mechanical support.