A professional designer will know how to design so that your restaurant or bar will best appeal to your customers. Restaurants are known to be amongst the most technically difficult interiors to design. A trained restaurant designer will understand the technical matters but will also be trained in design history, science and about design tools (such as computers and complex software). They will have time to do your work thoroughly. An experienced designer will tell you honestly if your own ideas will work and if they will not they should not be afraid to tell you and explain why and to come up with other suggestions.
A professional designer will take time to learn about you and your business, where it is and what it is trying to achieve. They will help prepare a detailed design brief to record these things. This brief will help you to control and focus your ideas to stand the best chance of winning business from your competitors so that you are not just relying on your personal tastes and assumptions. For more information I have written a separate help sheet called How To Brief Us.