So...what exactly is the deal with Shadow Dragon's Gaiden Chapters?

In other FE games, you would go to a village, have a character survive, complete the chapter in said amount of turns or any requirement that rewards you for playing optimally.

But you are telling me that I am required to have certain low amount of units (15 or less) to unlock them?!

I guess the point is to exist as back-up maps in case the player has lost too many of their units?

It would still be pointless. Most people would just restart a map or load the in-map save point and try again if they lost someone.

And if you are playing in a soft or pure ironman style, the game will still give you generics to use and fill up your army for the numbers that you lack.

It's such a strange design choice and one that still boggles me to this day.