A healthy, competent adult can handle his or her own financial and business affairs. However, a life-changing event can make that impossible.
Have you considered who will manage your affairs if you have an accident, develop dementia, have a stroke? Will someone be able to act in your stead if you take a trip out of the country? If you cannot take care of your financial or medical needs, someone will need to act for you to pay your bills, interact with your bank, make financial and/or medical decisions, or complete other tasks.