Fat gives gelato body and a creamy feel, and it carries flavour. In milk gelato we usually work between 6% and 10%. During batch freezing, some fat globules join together and form a network that holds air and gives structure.
More is not better. Beyond a point the gelato becomes heavy, leaves a greasy mouthfeel and masks flavour, especially with fruit and chocolate. The right amount depends on the flavour, not on a fixed base recipe.