People use personal business letters whenever they want to communicate with an organization (or sometimes an individual) in a formal, professional manner. It may be to express their dissatisfaction or their happiness about the company or individiual.
A person can write this kind of letter to a company that sold a defective product. The person can include the expiration date of the product, the serial number and any other important information that the store would want to know. Likewise, if customer service was lacking, the person can include details regarding the salesperson and the time of visit to help the store manager pinpoint the problem employee and resolve the issue.
A personal business letter may also be used as a cover letter for a resume. The format of a cover letter is the same as that of a personal business letter.