The difference between the standard and developer license is that the developer license includes the Photoshop .psd files and the standard does not. The files are useful for web designers who wish to redesign the layout of the theme, as they contain all of the layout/color/size information for all of the pages used within the theme. You still need knowledge of .css in order to make use of the files, so if you do not, then you may not find a use for them. Thanks.
