To prove the charges. There have been enough cases of “she looks too young to be 18” where they were, in fact, 18. This database (which I thought was actually run by the FBI, but whatever) let’s them show that the images were of Jane Roe, born May 5 1996, and the images/material were produced between 2008-2010.
IOW, to provide proof beyond a reasonable doubt that they were underage.