FAQs for Beneficiaries Who Have a Payee
https://www.ssa.gov/payee/faqbene.htm#&sb=3&a0=-1

I think the receipts/records are to make sure the money is spent correctly on the person getting the benefits and not stolen or misused by the representative.

Frequently Asked Questions (FAQs) for Representative Payees
https://www.ssa.gov/payee/faqrep.htm#&sb=3&a0=-1