Most companies maintain their own accounting departments to prepare financial statements for executives and stockholders. Most people keep track of their own money with a couple of accounting companies, even if it's just keeping receipts in a shoe box.
However, accountant companies are still one of the most lucrative and hottest kinds of service suppliers. Pop over to these guys because there are lots of reasons for it.
Though a company may maintain its books and execute internal audits on a regular basis, there are instances when it's required by law that an external agency audit the books and make sure that financial reports are true.
This is particularly true with non-profit bureaus and people receiving government funding. At times such as this, just a professional company that's totally independent from the company in question can lawfully perform such audits.
Most companies employ only CPA licensed employees. These people have invested more time in college studying the appropriate procedures of maintaining and auditing a organization's books.
They also have spent some time working in the area to get familiar with accepted practices in the business and have obtained a written examination designed to check their understanding of applicable laws and practices which affect them.
Even companies that work for profit have discovered that having routine external audits performed raises their credibility with both owners and stockholders. This practice generates a certain amount of transparency and accountability which would be missing when nobody outside the firm were permitted to find the books.