the pro 14s and 15s are plenty fast and could be used as work horses for a lot (most office work I'd wager) with a KBM and an external display. the only issue is it'd be kinda hard to take calls without a headset, but that's a good reason to take your old iphone out of the drawer, right? ;)
The main use case I've heard is video transfers.
If you shoot videos longer than a few seconds on your iPhone, you will appreciate a way to get them off of your phone at a non-glacial pace.
For professionals shooting significant amounts of video, this is more than a convenience feature. Once your device's internal storage fills up with video, which does not take long at 4K60, now your "camera" is a paperweight until you can dump that footage.
Obviously, only a very small percentage of people are using their phones this way. But I think Apple really values that market.
I get that it would be cool. For the ipad i think it makes sense to do that, but your phone...? Let's get real. Fast charging over a standardized cable with 10gbs transfer speeds is gonna be just fine.