There's value (and power?) to the length of stay though. It's a good starting measure to see which countries can you just hop on a plane to without a visa, but after getting off that plane, what if you want to stay for a while? The UAE passport allows 30 days in Korea or Japan, while many other visa-exempt passport holders have 90 days. The Schengen zone is a special case where the entire group of countries together allow at most 90 days every 180 days. As a tourist traveling between those countries, it would be useful to not need to leave the EU every 3 months, and go back after another 3 months.