(Why) do I need a website?
A fair question. Even today, many people don't.
Beer, or a sit-down restaurant meal aren't something you can actually sell online. However, most students and an increasing number of older people with free time and high disposable incomes are making leisure choices online. Advertising costs are rising all the time, and print advertising is an inefficient way of reaching people outside the immediate area, whereas a website allows potential customers to find you from anywhere in the world.
A web address on your cards will allow those who have enjoyed their meal to recommend the place to friends by email. And, a map online, perhaps showing local parking, will allow people who don't yet know the place to find you easily.
Naturally, you know your business and clientele. If any of this seems to strike a bell, you should probably try an online advertising site (preferably from ijnx, of course).