The Photoshop .psd files are not required to run the theme itself, and they are optional additional files which come only with the developer license purchase. The files are useful for web designers who wish to redesign the layout of the theme, but you can still customize the standard theme without them. If you have knowledge of these files and how they are used, then there's no question as to why they are useful and why you would pay extra for them. If you don't think you will need them, or if you do not have knowledge of css then you should select the standard license package. Thanks.