With Windows Phone 8's greater reliance on the cloud and the inclusion of the Media Transfer Protocol allowing pictures, music, and video to be uploaded to your handset via USB, there's less of a need of a comprehensive client such as Zune PC (which is still needed for Windows Phone 7), but there is still a need. The cloud is not the answer for everyone, and there's a certain feeling of security when the full sized family pictures are sent over a cable.
At the same time, there was an elegance to Zune PC that made browsing, listening and transferring data feel organic, rather than manipulating a spreadsheet. Not everyone is comfortable working with directories, strict file structures, and spreadsheet-like lists of content. Windows Phone 8 feels a lot more technical when connected up to your PC.
You can download the client directly from Microsoft.