Problems of Guardianship or Control
With a trust, when minors are the beneficiaries the trustee can manage and invest the trust funds, free of the costs and restrictions that arise when a court must appoint a guardian of the property until the beneficiary comes of age.
Additionally, with a trust, you can continue the management of a beneficiary's assets to whatever age you desire; certainly beyond age 18 (the age at which ALL guardianships must terminate).