When designed effectively, a sign can combine with other media to help brand your business in the mind of a consumer. Text and images on the sign should be repeated throughout your marketing mix, either within your company (stationery, catalogs, business cards, uniforms, vehicles, etc.) or when advertising elsewhere (TV, radio, Yellow Pages).
The more consistent your message, the more reminders you give customers to remember who you are, and what you're selling. The goal of any business is "top-of-the-mind" awareness. If a consumer in your community was asked which company first comes to mind in your industry, would that person think of yours?
Some sign companies can work with you to design (or change) the graphic image or logo that represents your business.