In the case of Jack Gordon, missing since October 2, 2008, and other missing persons, we know who the individual is, we just don't know where they are or what happened to them. There is a flip side to this situation however. Sometimes a person is located, but their identity is unknown. This can occur when there is a living person, such as an amnesia sufferer, or a person in custody but willfully hiding their identity. It can also occur when an unidentified body is discovered (homicide, suicide, unknown cause of death without identification). Matching a found person to a missing person is the ultimate goal.
I recently stumbled across the website "Unidentified Persons- Gateway to the Unsolved". This site focuses on people who have been located but not identified. The site has state-by-state links to official resources, as well as pages of individual photos of either amnesia victims or deceased individuals, hoping somebody will see the picture and recognize the individual. There is a section devoted to victims tattoos and identifying marks, and a section with personal effects of unidentified persons.
It seems to me even more tragic when there are found individuals who are not identified. It may be that the person's loved ones have not been able to connect with the agency or website that has the key information. Perhaps the individual ended up in a jurisdiction far away from where they disappeared. Perhaps the time lag between going missing and being found is so large that the connection is not made. Clearing houses or compilation websites such as NAMUS are attempting to overcome some of those "needle in a haystack" obstacles. The sadder scenario is that nobody is even looking for that particular individual. I cannot fathom how far off the radar somebody must be to end up as a potentially identifiable and yet still forgotten person.