A consulting company or professional services firm basically 'rents out' their employees to other companies that may need help with a specific task (1) or can't do a task on their own for legal/business reasons (2).

1. Sometimes, a company just needs a specialist to help them achieve their goals with a specific project. Maybe they need the services of an appraiser, or an actuary, or a PR person, or a software developer, or a lawyer with a specific specialty, for a limited time. So, this company asks a consulting firm to 'borrow' someone with the specific skills they need.

2. Consulting agents are often hired by companies to do audits. A company can't really audit itself effectively, since everyone in the company has a vested interest in making themselves look good. Bringing in an auditor from outside allows a more neutral look into how a company is running. This is especially important if the results of the audit will be shared outside the company that requested the audit to be performed.