Why is service in a legal action so important? We have a constitutional right to due process. That means you have to receive notice of a legal action against you so that you have the right to respond and contest it. The way you get notice is by being personally served with a copy of the legal action. If you ignore that service, then the moving party can take a default judgement against you, receiving what they petitioned for. In Washington, the summons requires you to respond within 20 days (60 if you are being served outside of the state). If you appear, the moving party cannot take a default against you without giving you notice first. The proper way to appear is to file a Response to Petition with the court, and serve a copy on the petitioner or their attorney. Do not ignore service of process.
There are a variety of ways get service of process. The best way is to get personal service on the other party. The pleadings must be served by someone other than a party to the lawsuit, and they must be 18 years of age and of sound mind. They should file a Declaration of Service in the court file, verifying that it has been done. You can also get substituted service, by serving an adult who lives in the house with the respondent. In some cases, if you cannot locate the respondent, you can get court permission to serve them by mail or by publication in a local newspaper.
If the other party was not properly served and did not have legal notice, and you take a default, you run the risk of them coming back several months later and moving to vacate the default, and starting the action all over again.