The US legal system requires that all parties be notified of legal actions against them to satisfy the constitutional requirement of Due Process.
This affidavit acts as the evidence that the notification occurred. It details the date, time, and manner in which the documents were delivered, and identifies who received them. Without this affidavit filed in court, a judge may not be able to proceed with the case or issue a judgment, as there is no proof the defendant knows they are being sued. It is typically signed by a professional process server, a sheriff, or a disinterested third party over the age of 18.