[toni-a]

As said circular fisheye gives you more flexibility cropping than a diagonal fisheye however, I prefer diagonal fisheye.
I just finished shooting a church ceremony (50th wedding anniversary for 10 couples), I used my samyang 8mm on 7D2 not on RP
So yes at least for me I don't see a practical utility for circular fisheye, but people like numbers so a 4mm would sell better than a 6.5mm, and that's how I bought the 8mm when I was a 5D user.