These back-ups revealed origin stories for the team.
Another brief era had the main stories truncated to make room for back-up features. At one point, the team had been disbanded and the stories became solo adventures of the team. And given that X-Men was not a hot seller in those days, it wasn't unusual for a new gimmick to be utilized every so often to revive attention in the book. It wasn't until years later, after Roy Thomas had assumed control of the book, any real thought was given to the backstory of the X-Men.
