Are you talking about the oil slinger or the oil collector? The slinger is essentially just a larger washer and it sits on the input shaft that slings away oil. The collector cups around the front gears creating and oil channel directing lube on the meshed gears and it bolts to the front of the housing
There's no good reason to have either off not to other than you just are lazy and don't feel like putting it on. Too add to Oilly's post, you get low on the oil level and hit a steep long climb, you'll wish that collector was there.