Just because there is a lot of information on the site doesn't mean it will necessarily answer your assignment question. There are many controversial and biased viewpoints on the Internet. Appearances can be deceptive so they are not always good criteria to use. This does not mean that academic sites are good and commercial sites are bad. At the end of the day you must evaluate the contents but it can be helpful to know where the information is coming from.
Internet sites tend to be accessible at any time from any networked computer.
They are cheaper Not necessarily. This is one of the greatest strengths of the Internet. This is a great strength of may sites. Sites as well as archives can be easily searched by keywords. All the materials you need are there, with more quizzes and exercises to lighten the learning experience.