This is often called leaving documents with a person of "suitable age and discretion" (usually a competent adult or older teenager) who also lives in the same home.
This affidavit is crucial for proving that, even though the defendant didn't take the papers hand-to-hand, reasonable steps were taken to ensure they would receive them. The process server must attest that the home was indeed the defendant's usual place of abode and identify the person who accepted the documents (e.g., a spouse, parent, or roommate). Substituted service is generally a secondary option used after personal service has been attempted.