I suspect that the impetus behind this wasn't Snapchat thinking "Hey, we should add a payments feature!", so much as some Square bizdev guy convincing Snapchat

I suspect that the impetus behind this wasn't Snapchat thinking "Hey, we should add a payments feature!", so much as some Square bizdev guy convincing Snapchat to add payments. Payments is a highly competitive and commoditised market, which means that margins are under intense pressure (what I like to call a "Race to Zero"). The industry will "consolidate" around a few winners, and those winners will be the companies that successfully execute on the strategy of "Reduce the incremental cost per transaction to as close to zero as possible, and pump up the volume". Everyone is desperate for volume, so every payments company is desperately trying to strike up as many partnerships like this as they can. Any app or website that has lots of users is a prime target.