I did it because i wanted to try it.
I’ve found the following things to be useful, your mileage may vary:
1) When i put my bike in a bike bag to fly or travel, i can now easily unbolt and disconnect the derailure without having to phaff around reattaching and realigning a bowden cable when it goes back on.
2) The ability to adjust the indexing from the display in seconds is really handy when, like me, you have a couple of sets of different wheels that put the cassette in a slighly different offset to the frame. I just remember +3 for my DH wheels, and -2 for my winters and can then ride either set without any sifting noise or crunching
3) The “saver” mode where the system releases then resets after the dearilure gets a knock has definitely saved it a few times. In fact, in the Alps, i hit rock, the system disconnected then reset, and the guy behind me did exactly the same thing (same rock!) but broke his conventional mech because the system couldn’t release and move out of the way
4) I like being able to chose which button does UP and DOWN to suit how i like changing gear
5) Minor one really, but I like how clean the install looks, without cables flapping everywhere
Ok, none of those are a big deal, or worth the extra cash for someone who’d be better off spending the money on say a better fork, but once you’ve got all the best bits, and if you can afford the ££ then i think 1by di2 does have real advantages.