If you make a certain site your default page you are more likely going to use it for looking at their adverts or doing a search.
Web sites make money by showing adverts, or doing searches, so they WANT you to set their page as your default page.
Did you know, for example, that if you click on an advert one one web site and go to a web site like Dell, Argos etc then the web site you clicked FROM get a little bit of money (called click through).
http://en.wikipedia.org/wiki/Clickthrough_rate
If enough people do this then a company can make a fair bit of money.
This is probably where answerbank makes its money from.