It's not really necessary if you leave the parking brake on, but for $10 it's worth it to give a cleaner look, and it probably does keep water out a little better.
Leave the old unit on if you disconnect the brake. It keeps the dirt and water out of the back of the caliper and keep that piston seal from going bad on yah. That area was never made to collect dirt and dust. And rear calipers are expensive/pain in he ass to always have it rebuild when the seals go out.