The nubs are a backup system to keep the front wheel from falling off when the QR isn't properly tightened. In order to remove the wheel you have to back off the skewer nut after opening the QR.

As AEO said, these are often called lawyer tabs, though here in NY "lawyer's lips" is the more common name. IMO the irony of these is that they actually increase problems because if the user doesn't know how to properly use a QR, he can't be expected to properly re-adjust the QR each time after mounting a wheel? Many experienced riders file these off, and some, like myself, modify the nut so it spins with the skewer and reliably retains the adjustment putting the Quick back into quick release.