In particular there's a period where you get "strings" that aren't actually text as we'd understand it but instead a sequence of glyph numbers for a typeface. So instead of ASCII's "A" or Unicode's U+0041 LATIN CAPITAL LETTER A you're just encoding that you want whatever the typeface named "Typewriter Sans" has put in slot number 65. Maybe it's a capital A and maybe it isn't.
This works pretty well on one Windows PC, or even a LAN full of identically configured Windows PCs. You can see why it was at least superficially attractive to Windows application programmers. Got a niche that needs Bulgarian ? No problem, the software doesn't care what the glyphs "mean", just define how to type the codes in and you're done.
But then somebody tries to open the Excel file with the Bulgarian text in "Steve's Bulgarian" font on a Mac and it doesn't work, it's gibberish. Oh dear. You have to install "Steve's Bulgarian" on the Mac and it look better although it isn't quite exactly right.
Today this is obviously completely insane, but by then the problem is backwards compatibility.