The CBTS serve as an 'equalizer' in preparing everyone for the instructor led classes. The CBTS contain all the information that is in the manuals and that will be covered in class. The candidates take the computer based training at their own pace. Someone with experience and background in the material may complete them in a few hours. Someone who is new to the material may spend several weeks on the CBTS to prepare for the class.
Some people learn best when they are interactive with the material on a computer.
Some of the courseware is also available on videos. Candidates who are new to the material often benefit from seeing the material in video format before taking the instructor led class. Since each of the candidates have taken the CBTs, have reviewed the manuals and possibly seen the videos, they know what to expect when it is time to take the class. They all have at least a basic familiarity with the material and the instructor will be able to conduct a much better class than would be found at a traditional training center. Most of them also have many years of real-world experience to bring to the classes.