According to Small Business Trends these are the "5 Reasons Why Business Signs Work"

Location: For a brick and mortar small business your sign is important as it acts like a guidepost to guide customers to your place of business and attract the attention of new customers. If you are a new business, people driving by will be attracted to stop in and try your business. People moving to the area look for local businesses that they can start visiting.

Brand Promise: A sign should always be neat and bright and should convey the promise of your brand, convey the high standards of your business, your differentiation. A sign board that is damaged or smudged may fail to convey your brand.

Special Occasions: Find local events and community gatherings like parades to create awareness for your business.
Competition: When Groupon rented ad space outside Living Social’s offices in downtown Washington, D.C. it created some buzz. You see these signs often on highways advertising restaurants or gas stations just a few miles ahead of an exit ramp. Signage is important in a high traffic location as overflow traffic from other places can find your business and make an impromptu decision to use your business.
Online-Offline Connection: Adding your website url to your sign is a good idea for people who don’t have time to stop and can check you out later. Adding a QR code is a good idea if your sign can be scanned easily.
In photos of your business on your website, it may be a good idea to include pictures of your businesses physical location, signage and neighborhood. So that when people visit, they are already familiar with how the business and surroundings look.
More and more people are finding your business online, but that is only half the story. If you have a great location, you can also appeal to new people who are walking past your business every day.