Padding done by the bookbinder to lift the spine away from the sewn signatures of the book.

The simple answer is that it was done to recycle old papers or manuscript pages no one needed, rather than having to pay for new paper that no one would ever see anyway. I believe this is called text block lining, and it reinforces the text block that has been sown together (in other words, all the pages are sown together, and then paper is glued on to the end of that to reinforce it). Modern case bound books (hardcover, rather than paperback) still use more or less the same method, although I'm not in the bookbinding business and I don't know how often fresh paper is used instead of recycled nowadays.

It's very common to see old papers reused in this capacity, and it has a history going back a long ways. In fact, this is actually a fascinating topic, and I'm going to have to ask a follow up question, because I'm familiar with a very interesting study related to this, but now I'm having trouble finding it: I remember there being a study that looked at pages from medieval liturgy being reused in this capacity in later books. Is anyone familiar with that study?