All too often a person writing their own will uses a word that means something significant to the legal system and very different from what the testator intended. Also, a layman may not consider a particular set of facts that actually occur. For example, a holographic will may say I want everything to go to my brother Harvey, but does not explain what happens if Harvey predeceases. An attested will is a typed will witnesses by two witnesses. Typically these have been prepared in an attorney's office, although there are numerous forms around.
Some forms are quite good.
But even then the testator has to make certain that it is properly adapted for his or her circumstances. The Cadillac is a self proved will. In addition to having two witnesses, the testator and the witnesses signed an affidavit which is attached to the will. This eliminates the need to find the witnesses and to bring them to court. Even if the witnesses are dead, the will can be proved up without fuss or muss.