A computer can be dynamic. Whereas a diagram or a graph on a written page is static. On a computer screen it can be animated or built up in stages. With a graph plotting package, for example, different parameters can be entered and experimentation can take place. Also investigations can be designed, all of which stimulate interest and bring about understanding.
Computers can also be interactive.
When students are learning from a book, they are in complete charge of the learning process. In general the only action is on the student's part. With CAL the computer takes part in the action. The student might be asked to build up a diagram, to give the answer to a question, to change the input and watch the corresponding response or maybe to match up certain relevant items. In each of these cases there is an interaction between the student and the computer.