Your market changes constantly. Newcomers to your area means new customers to reach. People earn more money, which means changes in lifestyles and buying habits. The shopper who wouldn't consider your business a few years ago may be a prime customer now. Shoppers don't have store loyalty they once had.
Shoppers have mobility and freedom of choice.
Your doors are open. Employees are on the payroll. Even the slowest days produce sales. As long as you're in business, you've got overhead to meet, and new people to reach. Advertising gives you a long term advantage over competitors who cut back or cancel advertising.