Running your own business gives you the flexibility to do something that you are passionate about. You can take only the work you want to do, and at the time you want to do it. You'll sacrifice some potential income if you want to take the winter off, but in retirement you may not need as much income as when you had a family depending on you. There are many reasons why you might want to be your own boss in retirement.
Earning a small salary during retirement is the equivalent of having a larger retirement nest egg to live off of. It reduces or even eliminates the need to dip into retirement investments by providing money to pay day-to-day expenses, or to offset the costs of a trip or other big ticket items. If you're worried about whether you have enough saved up to live off of for the rest of your life, a retirement job provides a cushion to ease your mind.