Several reasons drive the use of Glycerine in brewing wine and spirits:

Smoothness and Mellowing: Glycerine smooths and mellows spirits, reduces their harshness, and improves their mouthfeel.
Reduces Ageing Time: It can help round out the flavour and reduce the ageing time of the spirit.
Enhances Texture and Mouthfeel: In wine, glycerine is a by-product of the fermentation process that can enhance the texture and mouthfeel of the wine.
Balances Tannins and Acids: It adds body and sweetness, and helps to balance tannins and acids in the wine.
Pairs Well with Oak Chips and Essences: Glycerine combines really well with oak chips and any essences used.
Adds a Touch of Sweetness: Glycerine can also add a hint of sweetness to the spirit.

Dose rate

Wine & Liqueurs: The general dose is to add 25ml to the finished wine or liqueur to help smooth the taste. Mix the contents well and taste. You can add more if you need a smoother wine.
Spirits: Typically, 5 ml per litre gives a good result. It’s important not to add too much glycerine, as it can turn the spirit soapy if the spirit is high proof.