What are the benefits of a Apple Watch ultra over the Apple Watch series 8

The biggest reasons I moved to Apple Watch Ultra are:

1. Brighter Screen
2. Larger Screen
3. Significantly longer lasting battery
4. Significantly enhanced durability
5. Better water and dust resistance
6. It's simply cool looking (as long as your wrist can support a watch which is 49mm)

it's more rugged I guess

you can see them compared here Apple Watch - Compare Models - Apple

a big plus to me is that you can set it up and use it as a Dive Computer.