There are many reasons but mainly are three.
PROTECTION: You want to protect your assets and inheritors. You want to give all assets securely after death without delays and legal issues.
TAX EFFICIENCY: You believe that your wealth is very important to you as you spend years in creating and building. So, it must have pathways after death to go to the right hands without tax implications.
OBLIGATION: It's not only a legal obligation but a moral and religious obligation. Legal because the government does not want to spend time and energy while you are not alive. Moral because it matters for your inheritors. Religious because estate distribution comes from inheritance or succession laws for Muslims, Jewish, Hindus, and others.