A PNG file contains image related data. This data can be stored in a multitude of different ways and the file size will vary for each storage method—even with the same raw image data.
PNGenie optimizes any given PNG file—trying out different methods of compression—with a goal of achieving the smallest file size. Smaller files are good for a number of number practical reasons as well as, obviously, saving disk space. Let's say you want to reduce the size of data files belonging to an iPhone application, or you want to speed up loading of a website that uses PNG images, for example. Resource forks, gamma values and software-specific tags are automatically stripped, too.