https://www.sfgate.com/travel/article/why-passengers-choosing-socal-airport-ontario-21244531.php Any big city, especially a large metropolitan area can always benefit from having a 2nd or 3rd major airport.
https://en.wikipedia.org/wiki/Ontario_International_Airport
https://en.wikipedia.org/wiki/Ontario_International_Airport#Ground_transportation
https://www.flyontario.com/at-airport/terminal-maps#
https://en.wikipedia.org/wiki/Ontario,_California#Transportation