PSD files allows you to edit your own variation of the image for example, in the Caitlyn one her underwear is removable, or you could undress only one of either at a time in the Girls Frontline set ,etc.
but still, as I mentioned, the PSD files has a lower resolution, meaning you cannot get as good quality as the original images and the bonus images do not include their PSD files
honestly this kind of upsets me and makes me feel kind of scammed, because I paid extra $5 for those files and wanted to edit those images myself