Speaking of alcohol, let's get a little more specific and talk cocktails. Fifty years ago, it would have been unusual to go over to your friends house and ask for a glass of chardonnay or a beer. If you were my granny, for example, you would have requested a brandy Manhattan to take the edge off. That was the norm.
Wine is much more prevalent now as a cocktail hour beverage. However, mixed drinks are definitely experiencing a resurgence in popularity. We recently put together a nice, little bar cart area near our kitchen and have been slowly stocking it with miscellaneous liquors, liqueurs, bitters, garnishes, mixers, juices, etc. It's like our boozy little science station!
Anyway, it's certainly not necessary to have a professional, fully-stocked bar situation. Yet, there are some basics (beyond the wine, beer and soda you have in the fridge) worth buying to get your home bar off to a strong start:
1) Liquor: Gin, Vodka, Tequila and Bourbon are good basics to begin with. Add in Scotch and Rum if you / your friends are particularly fond.
2) Liqueur: I like having Chambord (a raspberry liqueur) for making Kir Imperials (champagne + chambord), but that's just me. Kahlua (coffee flavored stuff) isn't my favorite, but I know a few who love it in White Russians. St. Germaine is in many a cocktail recipe, and also good with sparkling wine. Really, there are so many liqueurs it's all up to what your tastes are. You certainly don't need them, but a couple are nice to have.

3) Fortified Wines: Vermouth is essential for making martinis and Manhattans, among other classic cocktails. Port, Sherry or Brandy are other good options for after-dinner sipping... but pick one, if you're just getting started. Or none. Vermouth is most important.
4) Bitters: Again, there are so many varieties. Get a bottle of Angostura bitters (used in Manhattans and Old Fashioneds, among others) and go from there.
5) Mixers: The basics are tonic, club soda, and ginger ale. I'd also add in some Rose's Lime, for making gimlets!
6) Garnishes: Maraschino cherries and olives. Also, obviously, lemons and limes.
Bottoms up!