Books on CDs are in digital format and thus, take up negligible physical space as a CD's dimensions are much smaller than those of just the front cover of most books. This allows for a high level of portability and virtually eliminates the physical storage constraint. Imagine stuffing the entire encyclopedia Britannica into your shoulder bag; the whole encyclopedia can fit on a few CDs or a single DVD. Most books however, are small enough in digital format to occupy only a fraction of the data space available on a CD and so a single CD can have a collection of books. In fact, text-based books---as opposed to audio books---take up so little space that a small library of these can fit on a single CD.
Conventional books can only offer text or images, which can only depict a certain scenario or the subject matter to an extent. Books in digital format however, can feature multimedia including sound and videos, enhancing the reading experience.
Printing costs are the major contributing factors in the cost of conventional books. A printing press is costly to set up and to run. Prices of ink and paper have to be considered. This cost often severely limits the number of colored pages and images found in books, and the high cost of books featuring such images. A further cost is the binding cost, which is at times reduced by several publishers by using lower quality binding---resulting in many books falling apart with repeated use. In the case of digital books, there is no printing or binding involved and no use of inks. Thus, colors and high-resolution images can be used without affecting cost.
Since books on CDs are in digital format, they can be copied from the CD to the computer and also transferred to portable devices for reading on the go. Devices built for viewing electronic books called eBook readers are becoming increasingly popular due to their convenience. Many of these devices use a technology called E Ink to simulate printed paper and to provide a reading experience similar to conventional books.
Since electronic books on CDs don't involve any printing, no trees are used in their production. Thus, these books play a part in decreasing global warming and protecting the habitats of threatened or endangered species.