Whole cluster (or bunch) fermentation is the process by which everything goes into the fermentation process. Skins, seeds, stems, you name it. The invention of the destemmer revolutionized wine, but could you imagine Burgundian winemakers in the 1800s hand picking each berry to get all the stems off? It’d be impossible, and the grapes would go bad long before a wine was produced. This method is the way wine had been made for a very, very long time.
We reference Burgundy in the example because you still find quite a few producers using this practice in the region, but it is not required by law, nor is it 100% of the grapes when done this way. Some producers don’t do any whole clusters, some 50%, some 90%, it all depends on the winemaker, vintage, desired outcome…honestly, any number of factors. The practice is also found with Syrah producers, so it is not exclusive to Pinot Noirs. You can find the practice in the old world and the new world alike as well. The practice can be done in closed containers and open top fermentation as well.
The stems do impart certain flavors that would not be there if it were not for their inclusion. Sometimes they can add tannins (but some debate this point), but they definitely add a certain texture to the wine that it wouldn’t have otherwise. Since the stems have no sugar, but hold water content, they don’t increase the alcohol at the end of fermentation, but can add a certain je ne sais quoi that is recognizable to the more refined palates out there, but for less refined palates, can make for a much more enjoyable experience with a varietal they might already feel a comfortable association with.