Most businesses require the services of an independent contractor / sub-contractor or consultant, which is not an employee of the business. For example, if you offer advertising services, but your expertise is not in digital advertising, but you still require digital advertising services, you might decide to appoint an independent contractor to provide digital marketing services to your clients.
Alternatively, you might be the principal contractor providing certain services to your clients which you would like to outsource to a sub-contractor or consultant.