If you are serving something common, then this may not be entirely necessary (though still cute and fun), but if you are serving something unique or complex, then signs may be helpful.
Most of the time, people recognize water, tea, and lemonade, but if you have beverages that are different a sign can be helpful when choosing a drink.
For a full blown meal, a menu is handy so guests can start making their dinner choices early to help the catering go off without a hitch.
If the guests have to go through a line to build their own meal, an idea of what's ahead can help smooth the food table flow.
When given multiple options where flavor is difficult to tell (like these yummy looking cupcakes), a little sign can do wonders to help guests make up their minds.
Lastly, and most important, if you are serving food with ingredients that people are commonly allergic to (like peanuts, milk, soy, fish, etc.) then displaying a sign with a short list of ingredients would be helpful to keep medical emergencies at bay. And if the food is complex then this would also give guests a better idea of the flavors.
And there you have it- a reason to make more signs! :) Wishing you all a happily ever after!