The term "business developer" is so broad it's almost useless. Anybody who does something to "develop" the business qualifies. If you're going to have a legitimate business development strategy, however, you're going to need a deeper understanding of what a business development manager truly does.
Therefore, it's more practical to replace the word "development" with "growth." Businesses need to grow. To accomplish that, they seek new customers and new markets. They bring out new products. They establish relationships that can bring them new opportunities to grow by carrying out joint ventures.
Therefore, a business development manager is partly a sales person, partly a marketer, partly a strategist, partly a business coach and partly the public face of the business.
Law firms often have partners who make a lot of money even though they haven't opened a law book in years. They have the contacts and relationships to bring in lots of new business. That makes them more important than mere legal experts. They're called rainmakers.
Good business development managers are rainmakers. They guide a business to keep growing in a sustainable direction and in a sustainable way.
Small and medium-sized businesses should not concern themselves with hiring business developers. Those roles fall on the shoulders of the founder and CEO, and to other high-level executives. If they find venture capital or an angel investor, that person will help them.
Unless a business is already large and profitable, many good business development managers will not want to help out except as paid consultants or by getting a split of the profits. The CEO and other shareholders may not be willing to trade away a portion of the company at that point in its life. However, the business developer is really good, it could be a good arrangement for everybody.
Some copywriters are willing to use their writing and marketing expertise to help a company grow for a share of the profits. They can also help guide the company in coming out with new, profitable products.
Once your business has grown, you may be able to hire a business development manager with the broad authority to get customer feedback through sales people, to help guide marketing, to bring out new products and to reach out to other companies to set up joint ventures. And some pieces of the process may usefully be obtained from consultants.
But the ultimate responsibility will remain with the owners and the CEO.