Anyone still wondering what the hell NIP-57 is, here's an explanation: https://github.com/nostr-protocol/nips/blob/master/57.md

TL;DR:

This NIP defines a new note type called a lightning zap of kind 9735. These represent paid lightning invoice receipts sent by a lightning node called the zapper. We also define another note type of kind 9734 which are zap request notes.

Having lightning receipts on nostr allows clients to display lightning payments from entities on the network. These can be used for fun or for spam deterrence.