The thing with these free domains?

I'm sure most of you know sites like:
http://www.unonic.com/
http://www.joynic.com/

But I just don't get it. What benefit they have from giving out free domains + hosting? Will they try to steal your files or infect your computer?
I'm using some of them for domain redirect to a offer I'm promoting.

Does anyone of you have a little bit more information and do you have experience with similar websites?


Thanks.