It’s customary, that’s why >99.99% of people do it. They don’t stop and think about it, or they would realise that it was typically a bad idea.

I would much rather have your custom font and then sans-serif, with nothing in between. Let me use your web font, or use my sans-serif, which is probably one of the fonts you listed in your fallback order anyway, but this way if I’ve expressed my own preference (I have) then these other Helveticas and Arials don’t get in the way.

The real problem with the fallback stack is that there’s no way of adjusting things depending on the font used. Perhaps web font A is about 10% larger than system font B, you really want to be able to say “bump the font size and line height by 10% if you have to fall back to B”; but you can’t.

So for simple fallbacks (of the “if this font is unavailable” type, rather than the “if this glyph is unavailable in this font” type), the main reason to add fonts between your font and serif/sans-serif is to match sizes better.

Then in the case of monospace, the problem is that some (most?) platforms have a default that most find ugly (e.g. Courier New), so I can more readily understand people preferring to opt into prettier fonts offered by the platforms, even if I wish they wouldn’t do it.

Perhaps they did it in order to make it very obvious when fallback has occurred, for illustration purposes.

Not a bad way to see it IMHO.