Rule 121 Answer Is Appearance

When someone who has been sued files an answer with the court, that counts as them officially showing up in the case. They do not need to be formally served with court papers after that point.

This means that once a defendant files an answer, the court knows they are aware of the lawsuit and are taking part in it. The whole point of serving someone with papers is to make sure they know about the case, and filing an answer proves they already do.