I'm no expert on Wicca but wine can represent many things, and is commonly used in rituals to symbolize things such as the bounty a goddess gives her children.
The House of Night series, I believe that's what you're reading, I noticed was incredulously accurate when it came the ritual set up (set up, not necessarily the immediate effect it sometimes had).
Regarding the ritualistic practice of offerings to gods, even in Wicca gods enjoy the concept of an offering, it shows initiative, and true devotion to them, and that the performance is not just for show. An offering requires thought, if you consider it special, then your deity will know. They will take into consideration your desire to please them. Wine is such a widely used symbol that it is what you make of it.
Also it is very traditional to offer wine, it's been a habit in rituals for centuries.