You will always love your kids, but that does not mean that you must trust them. When children grow into adults, they sometimes go their own directions. Some adult children never develop good money management skills. They may spend money as soon as they receive it, they may have significant debts or they may use their money to feed unhealthy addictions like drugs or gambling.
After you pass away, a single person will be in charge of handing out the inheritance that your beneficiaries receive. Even those who pass without a will would rely on this party to manage their estate properly.