The purpose becomes clearer when other people open channels too. So if I also had a tab nostr:npub1lrnvvs6z78s9yjqxxr38uyqkmn34lsaxznnqgd877j4z2qej3j5s09qnw5 now if you wanted to pay me you could go through him instead of having to open a channel through me directly.

So if you want to be me 5 sats you’d debit 5 sats from your “tab” and corn would “forward” that and debit 5 from his tab with me. He’d have the 5 you sent and I would have the 5 he sent me so corn would be whole (no change in how much is on his side).

Now how we keep corn honest (and not keep the 5 sats for himself) goes into more complicated “fairness protocol”.