The Photoshop files contain details of the theme layout such as colors, sizes (width of all the pages, header bars, sidebars, boxes, etc), for every type of page available, so when a web designer is looking to make changes to to the theme layout, all this information is readily available. Yes, they could potentially collect this information themselves, however it can be time consuming. It's ultimately your decision if you want to purchase the developer edition in order to have access to the Photoshop .psd files, but you can always upgrade from a standard to developer license at a later time if you need to anyway. Thanks.
