NWC is a protocol that connects apps to wallets and supports paying invoices and paying keysend. Wallets can choose to support one or the other or preferably both.
https://github.com/nostr-protocol/nips/blob/master/47.md#pay_invoice
https://github.com/nostr-protocol/nips/blob/master/47.md#pay_keysend
The reason we prefer keysend is because it doesn't require a bunch of extra LNURL/HTTP infrastructure to make it work. Your wallet pays another wallet directly on the Lightning Network and that's it.