External audits ar expensive, so are only done when they need to by the business.
The primary reason for having one is to ensure credibility for all the accounts of the business.
Consider a firm where the managers get rewarded from either selling it or having a high share value. It is plausible that they could "cook the books" to make the company appear one way when this is in fact not the case.
Only an external party can consider the accounts objectively vs management, hence why it is required.