One advantage is that the business owner does not have to pay for an employee, another is that eventually the business owner will learn how to be an accountant, which is a necessary skill for some jobs.
There are many disadvantages of doing it yourself, depending on what kind of business it is, you will have more things to put into the company’s books and if you don’t know how to properly it make take you more time then necessary. 

Hope you have a nice day!