Information is what your customers want, and it is what you want them to have.  The faster and easier that they find what they need, the easier it will be to get them back.  You can also guide them through your establishment the way that you might want them to.  Highlighting the items that you would want them to take a look at.  Did someone say specials?  Even if they didn't, your sign would.  A great sign scheme will be that employee that is always telling your customer about what you want them to hear.  A good sign can show off your past accomplishments, direct attention to high margin products, highlight amenities at your business, let customers know how to get hold of you, and so many other things.