Why would you take someone's DNA or implant a chip in them for identification, if their birthdate was not already known? If you don't already know WHO the pers

Why would you take someone's DNA or implant a chip in them for identification, if their birthdate was not already known? If you don't already know WHO the person is, you are verifying sameness of a person not relating to a known identity in identification. This is a real question. In what scenario would you chip someone or take their DNA if you didn't already know their DOB?