Those files are empty files. Normally, the skaters are packed in those files. When the game sees that they are empty, it defaults to the folder location for the skaters (Tony Hawk's Underground 2\Game\Data\models\). Then custom skaters in those folders show up in game.
If you are trying to add custom skaters, follow the instructions included in the custom skater that you downloaded. They should include a README or something similar telling you where to put the files.
