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

"...Sanders set up their ESL's at shows diagonally..."

And well out from any walls. They had the best sound at The Show.  

100% in agreement @russ69 (I heard them at the Irvine Show, two different years. Best sound at both shows imo). To hear them is to want them! I first heard of Roger when he wrote his multi-part article on ESL’s for J. Gordon Holt, published in Stereophile in the very-early 70’s iirc.

It goes against the grain - yet:  I have actually heard this once.  It seemed absolutely fine.  There is tons of great advice on speaker placement - I think all of it is well meaning and useful.  My humble two cents* just keep fiddling and experimenting!

* hoping you don't have heavy and spiked speakers!

This post and replies just set my agenda for the day. Thank you!

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.