I mean they don't even allow anyone else to connect to their lightning nodes.
[edit] oh I get it, they don't want to register as money transmitters haha, it's just to avoid regulation.
It’s using the Lightning Network to replace traditional interbank networks like Swift. Or ACH (at least for outgoing payments) If you don’t see this as a valuable thing, I’m sorry. It also allows arbitrary payments to lightning invoices denominated in USD, you can pay any user of the app, or any lightning invoices.
You don’t need to connect to lightning nodes - it is a network and if you need to route through them, it will.
Why bitcoin? Because they don’t need to maintain a liquidity pools and use swift to balance the USD pools. They can use cheap LN payments to instead of swift for settlement. Note that LN is unfairly cheap to use. An order cheaper than swift.
It’s indistinguishable from cash app to the user. Except that it works internationally and very low fees. It’s better solution than cash app but with crypto (abstracted from the user).