Without a properly executed will in place when you die, your estate will be distributed according to state law. If you are single, most states provide that your estate will go to your children, parents, or other living relatives. If you have absolutely no living relatives, then your estate will go to the state. This may not be what you want to have happen to your assets. You may have charities, close friends, or particular relatives that you want to provide for after your death.
If you become incapacitated without any planning, a court will have to determine who will have the authority to handle your finances and make health care decisions for you. The court may not choose the person you would have chosen. In addition, going to court to set up guardianship is time-consuming and expensive. With proper planning, you can execute a power of attorney and a health care proxy, which gives the people you choose the authority to act on your behalf, as well as an advance directive giving instructions on what type of care you would like. The power of attorney can also dictate exactly what powers the individual has.
Single individuals who are divorced need to make especially certain that the beneficiary designations on their IRAs, life insurance policies, and relevant bank accounts are up to date. If you don't, your ex-spouse could get the funds. And for single people of means, opportunities to avoid state or federal estate taxes can be more limited than for married couples, although advance planning can close the gap.
In short, proper planning is a good idea for everyone. Contact your attorney to help you create an estate plan.