It is required under the federal Servicemembers Civil Relief Act (SCRA). The purpose of this affidavit is to swear that the defendant is not currently on active duty in the US military.
Because active-duty personnel have special protections against being sued while they are deployed or serving, courts require plaintiffs to verify a defendant's military status before entering a judgment against them in their absence. The affiant must typically check the Department of Defense Manpower Data Center (DMDC) database or have personal knowledge of the defendant's civilian status before signing.