Insurance - to determine and enforce policy coverage.
Insurance Claims - to recover true value.
IRS - to determine state and local taxes, including estate and inheritance taxes.
Customs - to import & export vehicles.
Charitable Contributions - to substantiate value.
Marital Disputes - to guarantee an equitable settlement.
Pre-Sale/Pre-Purchase - to determine th true value of the vehicle you're selling or buying.
Restoration - to determine if that proposed restoration will return full value.
Many other reasons: gifts, business, personal settlements, etc.