If you keep someone on board with the intention of letting them go later, you do them a disservice by making them think they have a steady job. It stops them from looking for something else and missing possible opportunities.

If you tell them you will fire them, then you do them a disservice wasting their time if you don't expect them to work anyway.

That's why a severance payment makes sense. Pay them what they would have been paid but don't make them work.