A finance manager allows for someone to have an "outside view" of the company, and can work with other managers, such as one over advertising, to boost the sales for a company overall.
Having a finance manager allows for someone to have an unbiased point of view, and can serve as a "real-truth" function. Obviously, as a business owner, you are going to have some bias in favor of the company. A finance manager allows for someone who has no bias to the rest of the company and strictly focuses on where your money can be best spent.
Many entrepreneurs also have a great idea, but have little to no business experience. A finance manager can help them use their investments and sales to the best benefit of the company.
Overall, a financial manager can increase your revenue with their experience in properly spending money and their unbiased view of the company.