Why is the added liquid (seemingly) always alcoholic - as opposed to adding, say, water? Is it because beer or wine complements the other flavors in the fondue, or because milk proteins are alcohol soluble so it achieves the desired consistency? Or is there some other reason that combines those two explanations (or presents a different one entirely)?
The advertised reason is that the alcohol will cut some of the protein chains resulting in a fondue that is dippable and not so stringy. Obviously the alcoholic beverage of choice will also add a lot of cheese-compatible flavor as well.
Fondue recipes that don't include alcohol universally call for acid to achieve a similar effect.