It made more sense to go through the effort to install IrfanView when there was no image previewer built into windows, in the days of Windows 95/98/ME/2000. Those only had MS Paint, and I think some versions only supported bmp files (no jpeg or gif). Windows XP had an ok image previewer.
IIRC the killer "feature" that gave these previewers traction (ACDSee, IrfanView etc.) was that you could just preview a bunch of images in a folder using your arrow keys. So you'd just load one and use arrow keys to see the other images in the same folder. With the built-in options, you'd have to double click images one by one (and close their windows one by one) which was a horrible UX compared to what these provided.
That’s the “Preview pane” in explorer. It only supports the file types you could preview in explorer, it only “opens” the file currently selected in Explorer, and didn’t let you zoom in or inspect the image in any way that I recall. It was a plain preview that was supported (in ME) by the integrations Explorer had with Internet Explorer, I believe. Often installing IrfanView let you preview more file types in Explorer, and you could open more than one, display them full screen, edit them, resize them, and more…
