When you are in tablet mode and try to type 6 from the on-screen keyboard - the "-" sign is applied. 

When the RadNumericBox has focus it displays a clear button on the right, the "X".  In a high data density screen or grid, space can be at a premium and this button takes space, and when the user has a keyboard is not necessary (they can use the delete key) It would be space saving if this button was optional (as per the WPF version).