Placing a book in a protective enclosure (i.e. a custom-fitted box) is sometimes the best option. For example, if the book has a very special structure, but is too damaged to stand on the shelf, a box can protect it. A repair treatment would alter the original structure and obscure its cultural past. Since these books are read in the controlled environment of a special collections reading room, leaving the book in a weakened state is only a small risk.
The housing option also saves time and money: a book that is very rarely handled, but extremely damaged, can be protected while other treatments are done. This is why some kinds of housings are called “phase boxes:” the box is only a temporary phase before the book is treated. Other boxes are meant to be permanent homes for the books. Extremely small books were placed in boxes so they would not get lost on the shelf. And books with fragile materials, even if they are in good condition, can be placed in a box to protect them from adjacent books while shelved.