Grappa is a unique distillate in the world because it is obtained from the distillation of a solid raw material: pomace, which are the grape skins after they have been pressed to make wine.
The pomace that arrives at the distillery is essentially of two types: fermented and unfermented (virgin). The former contains an alcoholic percentage because it has fermented with the wine must, while the latter are almost always white and, except for rare exceptions, do not ferment with the wine must. Therefore, it will be up to the distiller to ferment them in the most appropriate way before distilling them.
Why is this fermentation necessary?
Fermentation is very important because it is the process by which an organic substance (usually sugar) is transformed into alcohol by unicellular microorganisms: yeasts. To create a distillate (such as Grappa, Cognac, or Whisky, etc...), you always need to start with a substance that already contains alcohol, that is, a fermented one.