Blockboards are considered better than plywood when it comes to making long book shelves or any other parts of furniture where the length of the boards has to span more than 6 or 7 feet.
The reason why blockboard is good for shelves is because its composition is very different from plywood. Unlike plywood it is not made from veneers (plies) glued over one another. Rather it is made up of solid rectangular blocks of wood (often called wooden strips or battens) that form its core.
The image below shows the insides of a blockboard.
Notice that the blocks are arranged end to end in a length-wise direction, and wood veneers (usually single layer of ply) are glued on both the front and back faces of the board such that the blocks inside get sandwiched between these layers.
Now coming to the question why blockboards are considered better than plywood for making shelves. Its because lengthy plywood sheets have a tendency to sag (bend) in the middle, while blockboards are stiffer and less likely to bend. This does not mean that blockboard as a material is stronger than plywood. Its usually not. In fact, blockboards are mostly made from softwood (pine wood core is very popular), whereas good quality plywood is mostly made from the stronger hardwood varieties of timber. Blockboard is only preferred in cases where long pieces of wood have to be used.
Hence for making furniture such as those lengthy book shelves lined with rows of books that you see in libraries, or for doors, partitions and wall panels, and for long tables and benches, blockboard is quite good. An added advantage is that it is also cheaper.
The selection of material also depends on how much load each book shelf is expected to carry. Another option is to use good quality solid wood (not blockboard) for making the book shelves. Such solid wood book shelves will be very strong but they will also turn out to be costlier.