This was a time when the majority of folks only had 14.4 kbs modems and 15 monitors that could display 256 colors. It was a rare treat to come across a page that had an image on it. Exceedingly rare to come across a page with multiple images. Video was out of the question. It simply took too much time to download the file.
Static images are all well and good but human beings like motion.
This addition to the GIF format allowed for motion. GIF animation relies upon a basic principle of how we perceive motion. If you take a series of still images and move them fast enough the result is moving pictures. Think of GIF animation as an electronic flip book. Many of us made one of those when we were a child.