It's just better. Seriously, I can't even use windows 8.1 after using 10. It makes you realize what an unorganized mess it was. Heck yesterday I did a WDRT reset back to the regular build and I realized how much better redstone is.
Biggest reasons(For redstone aka anniversary):
-Enhanced START screen
-Enhanced Action center
-More customizable: action center/start, etc.
-The settings are better organized and easier to use
-8.1 apps are being phased out. Eventually the backwards compatibility layer on w10m will be removed which will also make it more efficient.
-More apps showing up thanks to UWP.
-Integration with Desktop is improving
-Quiet hours can be used without Cortana/location being on
-Store is way better and easier to navigate.
-Flashlight button in action center. I have used it quite a few times and it's super convenient.
-keyboard track nimble is actually really useful when you get used to it. It becomes second nature to use it quickly for moving focus around.
I don't use Cortana much either. But now that Redstone doesn't require location to be on to use it, it's actually handy from time to time when in the car and wanting send a text or play music etc..
You can also set a different region and just change the language on Cortana.