Why do some sorbet and ice cream recipes call for the use of gelatin? What is the purpose for using this?

It gives the sorbet a softer, smoother texture at freezer temperatures, making it easier to scoop. (Gelatin binds with water and inhibits the formation of large ice crystals that make for a more granular texture.) Also gives a more viscous mouthfeel and helps scoops hold their shape better.

You can leave it out, but the texture and mouthfeel of the sorbet won't be quite the same.