Destem or not destem? This almost sounds like a title from Shakespeare. Historically in Chateauneuf-du-Pape, grapes upon arrival in the cellar are fermented in whole bunches, so not destemmed. The tannins present in the stems then combine those inside the grapes.
The stems, when ripe, give the wines a framework and structure, essential to the development of wines. Practice can sometimes be obstinate; "Many vineyards have a number of grape varieties and vines of varying ages. This makes it sometimes difficult to choose which grapes to destem and which not to." Sometimes winemakers destem partly creating different options.
Destemming can be important for some wines that already have enough tannin in the skins and seeds. In such instances, removing the stems makes the wine well-balanced, and not excessively tannic. When the stems are not removed, their degree of ripeness (lignifications) matters a great deal. Green stems can impart herbaceous notes and bitter tannins. However, when used correctly, stems can add interesting flavors and complexity. After this step the berries are (or partly) crushed and causes partial release of juices. The fermentation and maceration can begin !