In answer to unexpected market demand, or to speed up expansion into a new geography, sometimes stretching the existing teams further is impractical, as they are already fully engaged in on going activities.

Quickly augmenting the teams where and when needed is critical to business success, and especially customer satisfaction.

Projects come and go, they sometimes finish early, or get extended without much notice.

While maintaining the core competency of a company's own project experts, various project teams typically benefit from the time flexibility, cost effectiveness and capability mix of external project resources.

It is in the interest of the company to ensure that, whenever an employee goes on a scheduled temporary leave, be it pregnancy, education or public service, the "temp cover" is responsible, reliable and competent.

Imaginably there isn't usually an expert trumpet player in a string quartet, or a natural language processing specialist amongst a team of call center agents.

When the necessary skillset isn't present in the existing employee body, often the best and quickest approach is to seek the assistance of someone who "has got it".

Many functions are crucial to business continuity, and help ensure legal, financial and commercial compliance.  Yet they are not directly related to the core products and services of a company.

It could make sense to trial outsourced accounting, procurement, call center, warehouse, fleet management, HR administration, etc.