When making a simple syrup, i.e. 1 part water, 1 part sugar, you constantly need to brush the liquid on the sides of the pan to wash the sugar crystals back into the syrup. If there are any sugar crystals that aren't completely dissolved into the water, they will act as seeds that will cause crystallization of the syrup/ Adding corn syrup causes the sugar to completely liquefy, eliminating that crystallization.
When making your sorbet, the addition of the corn syrup does the same thing, it keeps the syrup component of the sorbet in a liquid state, again, elimination the formation of sugar crystals. This makes for a smoother, softer sorbet that will maintain good flavor and texture. The same thing can be done by adding agave syrup instead of corn syrup.l You can leave the corn syrup out of you sorbet recipe, but the results won;t be as good.
Seeeeeya' Chief Longwind of the North