I tell customers to go elsewhere to get a pencil.
Apart from stopping the leads from spilling out I don't think I have ever seen one being used.
Most drawings are done on confuser these days but we still had Drawing boards in the drawing office and occasionally they got used and when they did an eraser was a separate item in the tray with a rule init?
I don't think they are a requirement in a kit but I have only made a few too.