A little bit is about knuckles, not very much though. A bit more is about removing the "boxy" look.
Far and away the biggest reason for doing this is to protect the corner that isn't there anymore so now it won't need protecting.

The corner was very close to a right angle. The corners are now nowhere near a right angle. Right angles don't hold paint well and don't hold resin much better. They both tend to sag away from the corner whilst waiting for a cure. Anyways what with the occasional scuff from my paddle shaft and more so friction from tie down straps. (I know you can, and I do put padding there but it's still a sharp corner and gets a bit soft on hot days.) Both the webbing straps I use to tie down and various pads I've used to protect it have each taken a toll on the vulnerable "sharp" edge. I intend to put some type of wear pad onto the infill panel where the straps will go over it. The chance of me catching the paddle is much reduced, and all in all itsa good excuse to mess about with boats. In 'em or out of 'em it don't matter.
Mebbe I overtighten my tie down straps, but I don't wanna find out what is undertightening! Also Mick could probly comment on the difference between paddling a Laker and his Okwata in terms of THAT corner not being there anymore. In future I would never again affix a deck like that one having cut a section out of the join. So am learning stuff as well.
The varnish, paint and graphited bits are all due for a re-do so not a great drama to include this at the same time.
Regards John.