I never heard of this tip to draw at the end of the sketchbook, and it seems honestly like a weird idea to me? I don't know what's it supposed to achieve. But I like to do things in order
I do have a tradition, though, where I draw a group shot of my main OCs at the end of each sketchbook - it's kind of a way of measuring if/how much I've improved in the meantime. So I think it's cool to redraw an older work, or draw an image with a similar theme when you finish a sketchbook - gives you some feeling of achievement.
This is the newest one, from a few weeks ago; I've got all the previous ones in this tweet