My main reason is I work outside. I had a series 6 that I was just going to continue on with when the Ultra was announced. Then my job changed to working outdoors in the middle of winter. The standard watch is rated for around 30 degrees (F) on the lower end while the Ultra is rated for -4 degrees (F). With temps in Michigan getting lower than that I decided to get the watch that would handle the environments I was going to be working in. It’s also proven to be very rugged for all the banging around I’ve had to put it through as well. No regrets at all.
I use sleep focus specifically because it doesn’t light up when you move. You may be thinking of do not disturb.