A whole lot more custom watch faces, and I think a better UI. You can customize what screens are shown on the watch (like removing weather, heart rate, etc) also set specific actions to be taken based on how you tap the button on the watch.
Some of that may be available on the official app now. I haven't use it since the MI band 3.
I haven't used a Mi Band in years. Last one I had was the Band 2, and I found that app much later on its life cycle. Worked a treat for what support it had for the Mi Band though!