A company is a living, ever changing entity. As the company changes, so does the IT Infrastructure. Some systems become obsolete, some systems are disused and need to be maintained more regularly.
While IT personnel qualifications and expertise are very important, to ensure the continuity, safety and solidity of their IT Infrastructure, companies need to have their IT systems reviewed by an experienced third party periodically.
Much like a financial audit would delve deep into your finances, an IT audit will cover every aspect of your IT Infrastructure.