A holographic will is a will that a person writes by hand on their own, and personally signs it. Many states recognize holographic wills as legitimate. However, not all states do. Holographic wills do not require a witness, but they do require a signature, and proof that the testator actually created it.
Not everyone chooses to create a formal will with the help of a lawyer. Some people simply choose to hand write it. It could be because they realize they don't have much longer to live and they don't have another will. It could be because they are either soldiers at war, or sailors at sea and want to write a will just in case. There are many different reasons why a person will create a holographic will. Despite the fact that holographic wills are not exactly the same as formal wills, they still have the same legal powers in many places.