It is not necessary per se. Many remove it for appearance's sake. But keep in mind that there is an increased risk of damage to the bike of it is removed. You have to stay on top of your hanger alignment to be sure of not having your chain drop off the backside of your cassette and damage your spokes.

Two colloquial terms you'll sometimes here are "pie plate" and "dork disk". That latter is a little bit unfair, but it gets thrown around a lot. The part is there for a reason, and removing it, while visually appealing, does increase the risk of damage to the spokes.