You own a family business or a lot of rental real estate. If you become disabled or die, a trust makes it easy for your successor to pick up the reins and keep the business running. You want to avoid probate, the judicial process that validates your will and ensures that your property gets to the proper heirs.
Probate is controversial.
In many states, a lot of paperwork must be processed through the courts, and that runs up costs. You're exempt from probate because of the way you hold your property. No matter where you live, there's no probate for property held in joint names or property with a named beneficiary, such as an insurance policy or retirement plan. If this describes your life, relax. A will will do.