Times are tough for business right now and there is a temptation to keep marketing "in-house" to save money, but the learning curve and constant changes in marketing are staggering, and would challenge a busy entrepreneur to create a complete and effective marketing strategy. Even if the owner devoted the time necessary to learn all of these marketing elements, they would be tapped out with little time to run the business.
A professional marketing agency will boost sales/revenue, build the brand, protect online reputation and provide proof of performance while allowing the business owner to scale their business.