What’s the benefit of placing speakers like this? (diagonally at the corner)

Roger Sanders specifically prefers to set up his speakers this way because of the way first reflections are aimed away from the listening position. He is quite a character. For those who do not know him, he designed the curved panel for Martin Login. After deciding that was a silly idea he left and started his own company. His model 10 is quite a speaker but like my Acoustats very selfish. When you are in the groove They are quite a ride. 
The main problem with this approach other than décor are that it requires a large room. In smaller rooms the listening position is pushed up against the opposite side wall were you most definitely do not want to be. 
With a short wall system if you are careful with sound absorption at all the first reflection points around the speakers you will be fine above 250 Hz. It is below that is difficult. 2" thick absorptive materials only work above 250 Hz. Multiple subwoofers help but you still have between 250 and 80 Hz to contend with. My own solution is to use line source speakers that do not blast sound all over the room. I'm sure there are others that I have not looked into. I also designed my house with a media room that essentially has no back wall or rather a very scrambled one. 
If you have the room to do a corner system by all means try it. You just want to keep the listening position at least four feet off the opposite wall. The farther the better. 

Possibly of interest: room diagramming tool for measuring where things might go. https://sourceforge.net/projects/sweethome3d/

For those dismayed by the disjointed "look" of this setup, you could put an entertainment center type shelving unit diagonally in the corner.  It would create a false wall at the 45 deg angle, but would be symmetrical with the speakers.  Sonically it would not obstruct low frequencies, and it would diffuse upper frequencies.  ALSO... since the corner behind the shelves would be unusable dead space, you could put a large corner bass trap there.  This would be a great solution for a large room, where you wouldn't mind losing a few square feet to the corner.  

https://systems.audiogon.com/systems/3798

In my space, the diagonal allows for good symmetry when it comes to space from walls and listening chair distance. This allows for good soundstage balance (a very important factor for me). And the space that would be needed to set the room up asymmetrically in the room (far enough away from the side walls with a large open space on the other side) would be significantly more than a corner set up. 

I haven’t had my system set up in a rectangular room in years and I can’t remember the difference.  But here seems to be minimal smearing from side walls in my set up. I found that there was a much larger amount of interference from the speakers sitting too close together rather than too close to the side or front walls. 

So I suppose my sense of the benefit would be physical symmetry and use of space.