While roof contracting for the domestic, there was the odd one or two that wanted to assure my business and offered money up front.
For whatever reason, I did not like the money before the job was done and, the customer was satisfied. However, because I was being paid cash, the customer did not want the responsibility of having such a large amount lying around. Once I'm paid, they don't have the liability of losing it, ...somehow?
You will be right, in that, with your money being secured, the business can focus on the money that isn't. If you are not a regular buyer, you may well be suffering for this reason.
Upside? If the quality of the product has you smiling, then your money and your suffering supported something worth while.
...Ray.