There are four reasons for this. First, there is the requirement that the manager should be there only temporarily: doing interim management internally always means being biased by career perspectives.
Second, an external expert is free from the organisation's cultural influences and will be able to offer a fresh view of the problem. Independent professionals are listened to.
Third, the interim manager is aware of other companies' business practices and will be able to offer recommendations based on these experiences.
Fourth, employing an interim manager is a direct and clear signal to the organisation that the management is committed to solve a problem. Being an outsider enhances the interim manager's credibility.

Although there are exceptions to this rule, the interim manager will take up an existing position. If there is no existing position to fulfil it is considered project management rather than interim management. Interim management is temporary by definition. This temporariness has a number of advantages.
The interim manager is not required to gain credit within the company. He has nothing to fear from persistent antipathies that his decisions may bring about and that may prevent him from functioning normally after the changes have been realised. This allows the interim manager more freedom in achieving changes that are nearly always painful and difficult.