The Photoshop .psd files are useful for web designers who may be looking to change the layout of the theme as they contain all of the size/color/layout information already (and saves time collecting it). You still need knowledge of .css in order to make use of the files, so if you're not able to make use of them, it's likely not necessary to buy the developer license. You can always upgrade at a later time if you change your mind anyway. Thanks.