What is one meant to do with the water? Simply drink it after your coffee (hot chocolate) or if you prefer at the middle of it – it does not have any specific rule: instead it’s for your pleasure. Although to my experience they generally come with a sparkling (frizzante /frɪtˈzante/) shot of water rather than normal ones.
Beside the points @JoErNanO mentioned in his answer, sparkling water reduces the unpleasant (at least for others) coffee (chocolate) smell from mouth – so is a good idea to drink it at the end -, or as some say protect the esophagus from the acidity of the espresso, it also gives you a fresh feeling.
You may also find this article interesting.
The practice of accompanying an espresso with a glass of water is typical in southern Italy, and in particular in Naples in which coffee is something of a mystical ritual. Recently other establishments across Italy (and worldwide) have begun following this practice. High-end places might serve you a glass of water as an implicit statement saying that their coffee is so good that it requires you to clean your mouth to properly taste it.
The purpose of the glass of water is twofold.
This barista handbook (in Italian) explicitly mentions this:
Un ottimo servizio prevede che l’espresso venga accompagnato da un bicchiere con acqua (poca, non occorre pieno). Se l’acqua dell’acquedotto è buona, va benissimo.
Perché l’acqua? Perché il caffè fa venire sete e desiderio di bere. Quando berla? Un vero intenditore la beve sempre prima di bere il caffè, affinché rimangano a lungo in bocca il gusto e gli aromi.
There’s a chance that the bar you visited serves a glass of water with hot chocolate, for similar purposes as outlined above.
Of course, nothing stops you from drinking the water after finishing your hot beverage. You might see a few frowned faces around you, but all in all should you really care? I believe you’re free to consume your purchased drinks in whichever order you prefer, regardless of unwritten rules and traditions.
Credit:stackoverflow.com‘