In a well-planned estate planning execution, the trust, will, power for property, power for health care and changes to IRA and 401(k) beneficiary designations are signed and the deed(s) transferring real estate into the trust are all executed, witnessed and notarized on the same day. Then there is a period of time when financial accounts (checking, savings, brokerage) are re-titled in the name of the trust and the deed(s) are recorded. Let us call this time the gap period. If the grantor of the trust dies before all of the assets are re-titled in the name of the trust, then the will shall be probated so that the financial accounts will be transferred into the trust.
Without a will transferring all property to the trust at the death of the grantor of the trust, the property titled in the name of the grantor at his death will be transferred to the heirs at law of the grantor and not to the trust, where the property would have been distributed to the beneficiaries of the trust.
Likewise, it is possible that the grantor of trust could die without a will and even a year or two after his death the grantor could come into possession of property and without a will transferring property at his death into the trust, the late-acquired property would be distributed to the heirs of the decedent and not to his beneficiaries under the terms of the trust.
In short, a will is an essential part of a well designed estate plan.