Every sugar does two things: it sweetens (POD) and it lowers the freezing point (PAC). The catch is that they don’t all do both in the same proportion. Dextrose, for example, is about 70% as sweet as sucrose, but lowers the freezing point almost twice as much.
This means that by combining sugars you can set, separately, how sweet the gelato is and how soft it is at your display temperature. Sucrose alone rarely gets you there: the gelato ends up either too sweet or too hard.