For what tasks Netpbm images are used? Why 'pdfimages' use it as a default 'format' for extracted images?

> Normally, all images are written as PBM (for monochrome images), PGM (for grayscale images), or PPM (for color images) files. With this option, images in DCT format are saved as JPEG files. All non-DCT images are saved in PBM/PGM/PPM format as usual. (Inline images are always saved in PBM/PGM/PPM format.) > > — man pdfimages from Xpdf > The default output format is PBM (for monochrome images) or PPM for non-monochrome. The -png or -tiff options change to default output to PNG or TIFF respectively. If both -png and -tiff are specified, CMYK images will be written as TIFF and all other images will be written as PNG. In addition the -j, -jp2, and -jbig2 options will cause JPEG, JPEG2000, and JBIG2, respectively, images in the PDF file to be written in their native format. > > — man pdfimages from poppler What is the reason that both implementations of pdfimages extract images in "mysterious" (I call them "mysterious" because I have never heard of them before) PBM/PGM/PPM formats (collectively known as Netpbm or PNM, https://en.wikipedia.org/wiki/Netpbm) instead of PNG, JPEG or maybe GIF, which are (I might be wrong, of course) is the de-facto standard in casual-user-world these days (and, if I recall correctly, it was the same de-facto standard 10 and 20 years ago as well)?