Hard evidence on this practice is not readily available, but some scholars and reconstructive archaeology have provided some hypothesis as to why such behaviour was undertaken. They mostly stem from what understanding we possess of dining etiquette during the Late Middle Ages and the Renaissance.

By reconstructing feasts or just high status dinners/lunchs by adopting what depictions we possess (such as this XV century illustration of king Richard II of England during a banquet, or the numerous illustrations found in the book "Opera de l'Arte del Cucinare", "Work on the Art of Cookery" of 1570, authored by Bartolomeo Scappi (1500-1577) the cook of popes Pious IV and Pious V), the amount of dishes and servings could be astonishing. The presence of spices and condiments would also see an increase as the years progressed and the aristocracy managed to reinvest its wealth in what we might consider status symbols, like, again, spices and meals with expensive preparations.

Most likely to be placed in the later years of the XVI century, bleeding into the XVII, at least as Italy is concerned, the diffusion of beverages like coffee and tea might have increased the tendency to "correct" said beverage to the drinker's taste by adding sugar or spices to it, given however that said condiments were most likely present at the table within a single dish or vessel which had to be shared among either all the diners or just the three or four next to you. Since it seems that Medieval etiquette began to frown upon people drinking two at a time from the same cup, to the point of producing speculation that some surviving drinking glasses' rough texture might have been due to their usage with greasy hands, serving from the common spice dish might have undergone similar regulation.

Allegedly, drinking with your pinky finger raised might have been the norm because you might had to use only that finger (or a single appendage of your right hand) to take sugar, cinnamon, mace or whatever from the pot and add it to your drink.

I personally partly disagree with this theory, as we are aware of personal sets of cutlery brought by nobles themselves when they were invited over to another person's feast, or perhaps special sets of silverware could be produced for that special occasion (we might imagine for the king's son marriage for example).

I am a Medievist, so I cannot really give you an explanation about this well into the 1700s and later since it exits my field of expertise, so I would advise you to take this with a pinch of salt. Either with your pinky or not.