I'm not into it but I think I know why some are.
I was having bladder issues so the doc called for a cystoscope exam. The nurse inserted a tube with a camera into my cock and up my urethra to my bladder. She used a numbing fluid but still when the tube passed through my prostate I got an instant boner... damn what a feeling!
I could be wrong, but I think this is the goal they're after in sounding.
I've got a few prostate vibes so yeah I know what it feels like from the backside. The orgasms are totally mind blowing. It was just a totally different feeling sounding from the front... hard to describe