The Photoshop .psd 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. You can also always upgrade from standard to developer at a later date if you do need the files. In terms of theme functionality, there is no difference between one without the files and one with as the files are independent of the actual theme. Thanks.