Many times in addition to the life insurance proceeds (which is non-taxable to you) there is some interest income.

The interest income (if any) is taxable income to you, and the information on the W9 is used in case they need to issue you a 1099INT in the year the interest income is paid to you.

Businesses use IRS Form W-9, Request for Taxpayer Identification Number and Certification. 

Yes, I would fill it out, as it would speed up the payment of the life insurance to you.