Failures can cascade across the grid. A software bug in an Ohio utility's computers in 2003 ultimately took out power for 55 million people in several US states and the province of Ontario. https://en.wikipedia.org/wiki/Northeast_blackout_of_2003
If you have sites in multiple grids you're guaranteed that one failure doesn't take down every site.
If you have sites in multiple grids you're guaranteed that one failure doesn't take down every site.