Movie makers alter the book, Robot Dreams from the movie, iRobot, in order to make the movie different. They made the movie different in multiple ways. They made the movie different by making it more interesting, they unified the plot, and they controlled the budget.
The movie makes it more interesting to the viewers because of the special effects that were shown in the movie in order to make it look real. It lets people who read the book take interest, because they know the plot. Plus the movie has multiple stunt scenes where it attracts the audience. Because of these effects, the audience was entertained.
Movie makers made the movie different by unifying the plot. Robot Dreams have many short stories, but not all of them were displayed in the movie. A few were combined together to make the movie focus on one plot. If the movie makers put all of the plots in the movie, then the movie would be much longer. Budget would increase if this happened.
Producers of the movie, iRobot, had to shorten it from the book in order to control budget. If they transferred everything from the book to the movie, then the budget rate would increase dramatically. The movie would be really long if this happened. Plus the viewers will eventually lose interest of the movie, which would be bad for business.
Therefore, this is why movie makers had to make the movie, iRobot, different from the book, Robot Dreams. By making the movie more interesting, unifying the plot, and controlling budget, this made the movie different.