As mentioned by Rob, scratching the side of your head is very close to a covered position for a shielding elbow which covers your head, neck, eyes and face from someone on the side you are shielding.
Incidentally, I also use this on people that I perceive to be careless, uncoordinated, or exhibit erratic movement, particularly when they have a ranged object in their hands that might poke me in the eye (walking stick, umbrella, pointy toys swung around by kids, martial artists swinging their bo/tambos).
In the picture below I am isolating one element of a compressed cover I taught to the class, but you can see from this structure, that if Malcolm was to swing at me from where he's standing, having this shield up is an ideal position for me to take the hit or preferably slam into with a compressed structure, and it would be significantly easier to make this transition from an active grooming position with a readied hand/forearm than having my hands down, in my pockets, or clutching onto a phone. On a related note, slamming through someone's face, neck, chest with all of your body-weight focused on the points of your elbows is a pretty awesome way to get them moving back and get you into launching forward with strikes and pressure!
It is unnatural and draws unwanted attention to be walking around with a shielded elbow locked in, but keep in mind the principles that except in situations where clutching a tool is appropriate, unencumbered hands are easier to use and active hands are more quickly put into action.
Train smart & stay safe,
NOVA Self Defense