Receipts for the personal items that the airline lost, not for the airfare. You need to prove how much your damages are in order to make a claim.
In my case, I think they deliberately bumped my bag for paying cargo, because they saw my billing address was near the destination airport and assumed I would be fine "at home". They didn't know I was going to a wedding, and that said wedding was several hours drive away, so delivering my bag a day late to the destination airport wouldn't end the problem. They still paid out though.
You already need to insure higher values anyway if you want to be entitled to a reimbursement, so this would set a nice minimum that encourages customer friendly behavior.