It's not uncommon to drop down a steep descent, then suddenly hit a 25% wall you didn't know about. In such situations you're in a bit of a panic dumping cassette and hoping you don't stall out before you get a manageable gear. And fearing the comments of people behind you if force them to stack up behind you as you put a foot down. Electronic is quicker than mechanical, but it still takes several complete rotations of the crank to get the chain moved, and that's the real holdup. If you could get a huge change in ratio instantly and while under load, that's a pretty big thing.

Same with gravel and cyclocross. Being able to shift instantly and under full load in cyclocross would be an unambiguous benefit.

In short, getting a shift without the need to turn the crank to get the chain wrap completed is a real, measurable benefit. That's the big weakness of the FD and RD. We have shifting that moves the derailleur in milliseconds, but are limited by low torque, slow (relative to the derailleur movement speed) turns of the crank to get the chain actually moved.