Good idea; just make sure to check that Google Voice allows that kind of commercial use before hanging a business off it.
1) If you have live acts performing at your establishment Friday-Sunday, the website can raise awareness.
2) Specialty products / menu items.
3) Paraphernalia / SWAG from the restaurant.
4) Take out -- order online, pick up in store.
5) If you sell things like coffee beans, web presence increases the number of eyeballs on your product.
At the end of the day, restaurants are about putting bodies in chairs and serving them food. Thus, the focus of a good website should be increasing traffic into the restaurant.
One way I could see functionality between restaurant and website would be linking time-to-wait dynamically with the website. So if I go to a restaurant's website, I could see # of people waiting and average time to wait for the evening. This does go back to putting bodies in seats, but it would better link web presence with what actually happens in the restaurant.
I wonder if similar opportunities exist in the restaurant space.
When websites become more appreciated as a source of revenue then they will be taken seriously enough to actually improve. The big problem today is that whereas it's easy to show someone a competing restaurant and see how popular it is, it's a lot more difficult to see the effectiveness of other restaurants' websites since all of that data is more or less invisible to anyone but the owner.