It depends on if your question is why it is free to use or if it is free to submit content. It is free to use because of that a high amount of the Internets traffic does not like to pay money for...
Some of the good google groups include the health group, the business group, the news group, or the money group. You might also find interesting the recreation or the sci/tech groups.
I don't think there is any way for you to advertise for free in Google. The only way I can think of would be to SEO your site (white hat only - black hat gets you in trouble later) to try to get more...