Remakes and Originals
Like redrawing the entire game with HD 8 bit sprites? That would be way too laborious for Nintendo to bother with. Unless you mean just having the original game on it as a bonus.
Inclusion of the original game. Redrawing the whole thing doesn't do so much for that and thus circumvents the purpose.
With some of the changes that ended up being made (the fairy bottles, for instance), an actual toggle to switch between the versions might have been a bit more troublesome to implement, but I maintain the original and DX should have been included in some form.
the latter! you can say a lot of bad things about zero mission but it got this right
I certainly appreciate
Zero Mission's inclusion of the original
Metroid -- I have such a soft spot for that game, but I'd want it available regardless.
That's something I've mentioned for a hypothetical collection of mainline
Metroid games pre-
Dread: that the originals should also be included for those games which have received remakes. Not that I expect such a collection to be released.
Tenbōdai Course
As for the main topic, it seems plausible that this was an extra stage the developers put together -- whether simply because they could or because it fulfilled some purpose
(perhaps testing or some such) -- and decided to keep in the final product, not necessarily for any particular reason; perhaps they simply wanted to sneak something in.
It's actually a concept I'm rather fond of, that of additional levels or -- especially -- sub-games, especially those which might go without being discovered for some time or by the majority of players though this instance is right in the open).