This is known as the photic (light-induced) sneeze reflex. Somewhere around 30 percent of people are affected by this, but the exact way this works isn’t known that well. Most hypotheses assume that there is a nerve that carries signals from your eyes and irritates other nerves when it has to carry a lot of signals. The irritation of those nerves is what also happens when someone sticks a feather up your nose and you sneeze as a result of this.
Some people put their bread in the freezer and that setting both defrosts the bread and toasts it the normal amount. Generally the setting is useful if what you’re toasting isn’t already at room temperature for whatever reason, like if it’s a toaster pastry with frozen jam inside for another example.