First, it gives you much better picture of what mythic dungeons really are. At lower keystone level, dungeons mechanics can be avoided by your gear. You can only fully enjoy the content that game developers created for you in high mythic keys. Second, having the experience of high mythic keys will make you a king of +15 keys forever. You will just be laughing out loud how easy +15 key feels after doing let's say +22 in-time with our Pro Team. Finally, you also get increased number of items dropped while completing higher level keystones.