Why do I owe this person?

You may owe someone in the group who never paid for you.

You owe Ben 3 🍪.

Ben owes Anna 3 🍪.

So you give 3 🍪 to Anna, and Ben is done.

Without Knip

You → Ben3 🍪

Ben → Anna3 🍪

With Knip

You → Anna3 🍪

You pay the same amount. There are fewer payments.