Why is Unknown Soldier a symbol?

In the past this was an even more common phenomenon than today, however, often in the aftermath of battles soldiers cannot be identified for a number of reasons (Absence of identifying documents, Severe head or facial injury, Loss or destruction of unit records, etc...). Being aware of the unique tragedy that is represented by such a death, several nations honor their Unknown Soldiers with special monuments. The underlying theme is that no person who sacrifices their life for the good of the nation shall be forgotten or unmourned.