Automatic handling of different formats
A typical client's website contains some form of image or video archive. To view images they must be in a format that can be handled by web browsers, such as jpg or gif. Original images and videos are often in a different format and they must be converted to the required size and format. In addition, thumbnails and preview images are needed to more easily identify them.
Saves power for the web server
These conversions are demanding for web servers. It may take several seconds to convert a large image to a smaller size and converting video usually takes even longer. Conversions done by the web server can result in slow uploads for web visitors on pages where a conversion is underway. In comparison, the conversion of a large image in Photoshop locks everything else down on a computer.