Some apps used the disk image as a simple way to bundle up a little “installer”. This is lost by sending a .zip file.

The disk image could put a background image with instruction visual, decide the visual placement of icons. Applications could include folders with brush packs, example documents, user manuals and such.

The fact that it was a disk image could be seen as confusing if you read that closely, but if you ignore the file type it was simply an icon that you could double click to open a piece of installer UI.