You want to open a picture, FAST, no matter the format or resolution ?
You want to open a picture and then move from one picture to the next in the same folder with arrow keys or mouse scroll, again fast and without loading or menus fonctions or whatever ?
You want to batch process a folder to convert all files to png with the larger side limited to 2000px, keep the location data but reset the orientation data, and remove the original file only if conversion succeeded ?
You want to scan something, rotate it and lossy pixelize an area ?
You want to resize, convert, re-encode a picture from one format to another with tonnes of option without resorting to command line because you're on windows and you would like to just do it in the same app you use for every photo thing ?
You want to cut a part of a picture, or identify the pixel color on a picture, or dozens or other every day operations like that ?
You want all of that to be absurdly fast, aka instant, without any complex menu or dozens of clicks to get where you need ?
I've been using irfanview since the beginning too, and it's not for lack of trying other stuff, it's just so much better. It's for me one of those tools, like Everything or Ditto or SumatraPDF or 7zip or NAPS2 or ... That just get what they are and what they should provide, and do just that, and do it right.