This is a slightly sneaky way of saving money. The endpapers (shown in purple in this illustration) which are tipped to the first and last signatures of a hardback are the main way of holding the book block into the case. If you lay out the book so that there are blank leaves at the front and at the back, you can eliminate the endpapers and save the money by just pasting these blank leaves to the boards of the case.
The book will be a little weaker — the endpapers tend to be a heavier, stronger stock — though on the other hand the connection between the self end and the book block is more fundamental — the “endpapers” are part of the first and the last sigs, not just pasted to them.
Obviously using self ends means you can’t use any kind of fancy endpaper, colored as in this illustration, or printed with a useful map or whatever. We didn’t often do it, and when we did it was on smaller books on which last minute cost savings had become necessary.