blammo wrote I would assume it's to keep the stage volume down…usually your drummer determines the minimum volume of the rest of the band, & if he/she is an animal, then everyone else has to crank up their monitors and amps, creating a high risk of feedback. If the drummer is sonically isolated, everyone's mix can be controlled much easier. Or maybe he just smells bad.
Yes. On televised performances like the one you saw, the drums are typically well mic'ed. The shield allows the drummer to play at normal velocity, but with a reduced sound output onto the stage and into the audience. The other instruments were probably playing something softer. (It was the queen of Disco, after all).
You get a better mix of sound levels with the screen, as Blammo already said.