If this is allowable in the state in question, the benefit of doing so is that the list can be changed from time to time as opposed to changing or adding codicils to the will.
In California, Probate Code 6130 – 6132 state the requirements:
A writing in existence when a will is executed may be incorporated by reference if the language of the will manifests this intent and describes the writing sufficiently to permit its identification.
Probate Code 6131
A will may dispose of property by reference to acts and events that have significance apart from their effect upon the dispositions made by the will, whether the acts and events occur before or after the execution of the will or before or after the testator’s death. The execution or revocation of a will of another person is such an event.
Probate Code 6132
(a) Notwithstanding any other provision, a will may refer to a writing that directs disposition of tangible personal property not otherwise specifically disposed of by the will, except for money that is common coin or currency and property used primarily in a trade or business. A writing directing disposition of a testator’s tangible personal property is effective if all of the following conditions are satisfied:
(2) The writing is dated and is either in the handwriting of, or signed by, the testator.
(3) The writing describes the items and the recipients of the property with reasonable certainty.
(b) The failure of a writing to conform to the conditions described in paragraph (2) of subdivision (a) does not preclude the introduction of evidence of the existence of the testator’s intent regarding the disposition of tangible personal property as authorized by this section.
(c) The writing may be written or signed before or after the execution of the will and need not have significance apart from its effect upon the dispositions of property made by the will. A writing that meets the requirements of this section shall be given effect as if it were actually contained in the will itself, except that if any person designated to receive property in the writing dies before the testator, the property shall pass as further directed in the writing and, in the absence of any further directions, the disposition shall lapse.
(d) The testator may make subsequent handwritten or signed changes to any writing. If there is an inconsistent disposition of tangible personal property as between writings, the most recent writing controls.
(e) (1) If the writing directing disposition of tangible personal property omits a statement as to the date of its execution, and if the omission results in doubt whether its provisions or the provisions of another writing inconsistent with it are controlling, then the writing omitting the statement is invalid to the extent of its inconsistency unless the time of its execution is established to be after the date of execution of the other writing.
(2) If the writing directing disposition of tangible personal property omits a statement as to the date of its execution, and it is established that the testator lacked testamentary capacity at any time during which the writing may have been executed, the writing is invalid unless it is established that it was executed at a time when the testator had testamentary capacity.
(f) (1) Concurrent with the filing of the inventory and appraisal required by Section 8800, the personal representative shall also file the writing that directs disposition of the testator’s tangible personal property.
(2) Notwithstanding paragraph (1), if the writing has not been found or is not available at the time of the filing of the inventory and appraisal, the personal representative shall file the writing no later than 60 days prior to filing the petition for final distribution pursuant to Section 11640.
(g) The total value of tangible personal property identified and disposed of in the writing shall not exceed twenty-five thousand dollars ($25,000). If the value of an item of tangible personal property described in the writing exceeds five thousand dollars ($5,000), that item shall not be subject to this section and that item shall be disposed of pursuant to the remainder clause of the will. The value of an item of tangible personal property that is disposed of pursuant to the remainder clause of the will shall not be counted towards the twenty-five thousand dollar ($25,000) limit described in this subdivision.
(h) As used in this section, the following definitions shall apply:
(1) “Tangible personal property” means articles of personal or household use or ornament, including, but not limited to, furniture, furnishings, automobiles, boats, and jewelry, as well as precious metals in any tangible form, such as bullion or coins and articles held for investment purposes. The term “tangible personal property” does not mean real property, a mobilehome as defined in Section 798.3 of the Civil Code, intangible property, such as evidences of indebtedness, bank accounts and other monetary deposits, documents of title, or securities.
(2) “Common coin or currency” means the coins and currency of the United States that are legal tender for the payment of public and private debts, but does not include coins or currency kept or acquired for their historical, artistic, collectable, or investment value apart from their normal use as legal tender for payment.