The sun can overwhelm our vision and looking directly at it will damage our retina.  While driving this is a particular problem when traveling west at sunset or east at sunrise.  Blocking the current position of the setting or rising sun with your hand, or by lowering the visor, will help. Looking away so the sun is in your peripheral vision may help also. But all of these solutions block your needed vision and are knee-jerk reactions to intrusion.  They distract you from your driving.

All the metal surfaces of your vehicle and the glass and metal of nearby vehicles can reflect the sun into your eyes. This can cause multiple sources of glare, discomfort, and distraction.  Glasses can add to the problems by adding the artifacts including glare, flare, and star.