While standing in line for MO's at the grocery store, I am always puzzled by the people that use WU when paypal/venmo/etc is free (from a checking account). Maybe the people receiving the money don't have a checking account, or need the cash that very minute (not 2-3 business days later)? From a selfish point of view, these people irk me because WU transactions take 5-10mins and I'm nearly always stuck behind one of these people which eats into my hourly "MS wage" lol.
If you are MS'ing for cashback, its technically the same pot of money whether you are using it to pay your mortgage, the bar tab or a friend in another country. You won't find a payment system that allows you to send cash with a cc without a fee of ~2% which effectively negates any points/miles/cb you may have earned.