Consultants provide expertise that is not available in a particular organization.  As a rule, consultants are used for one or more of the following reasons.

Providing special knowledge and skill.
Consultants are called in when an organization does not have the internal expertise needed to tackle a given problem or issue with the same chance of success.  It may often involve new techniques and methods in which a consultant has acquired special expertise.  In other cases, the problem may be of a general nature.  If your organization is failing to achieve its principal purpose and the gaps in knowledge concern identifying objectives, developing strategies, management policy, planning, coordination or leadership.

Supplying intensive professional help on a temporary basis.
A thorough examination of major issues and concerns, such as the company organization or marketing policy, would require the full attention of senior management for long periods of time.  But the day-to-day running of the business leaves little time, and makes it difficult to concentrate on operational and conceptual problems simultaneously.  Consultants not only provide the time and in knowledge, but do not require a long-term financial obligation to hire in-house expertise.

Giving an impartial outside viewpoint.
Even the best people within an organization may be too influenced by their personal involvement and subjectivity to see a problem in its true light and propose feasible solutions.  Independent of the client organization, consultants provide an impartial objective viewpoint on affected by the day-to-day operations of the business.