This is really a marketing strategy. Give the software away, sell the content that most people will need to use the software. If DAZ studio cost $50 or $100 dollars, that would be a barrier to entry. Somebody who casually thinks he/she might be interested in try 3D drawing/animation, will be reluctant to pay money for a software they are not sure they will really use. They are much more likely to try it if the software if it is free and there is enough free content to let them try it. Now many of those people will lose interest and never buy any content, but a few who never would have bought the software in the first place, will buy content. Those are customers DAZ never would have gotten without the free software.
There also is other 3D drawing and animation software out there that is free, like blender. DAZ has to competing against that too.
You see similar strategies used in many businesses. Companies like HP, Canon, Epson, ... sell their printers at a loss and make their money selling ink or toner cartridges.
There are risks to this strategy. There is a lot of free content available, but most of the free content is built around some DAZ content that is not free, like the Evolution morphs or the Victoria, Michael, Stephannie figures.
The model may be slightly risky, but it’s tried and true - it goes all the way back to, of all things, disposable razor blades. I read in James Dunnigan’s book “Dirty Little Secrets of the 20th Century” some years ago that William (?) Gillette came up with the idea when marketing his first razor back in the 1910’s; sell the basic razor (with a couple of blades) cheap, and make the real money on blade refills.
I know that Daz Studio’s “free” price was the key point in getting me to try it; I don’t have a whole lot of money to toss around (who does, in today’s economy?) so if it’d been $100 or something like that as I think it used to be before February, I’d pretty much have been locked out from it. (Particularly since I’ve already spent that much or more on other PC software this year, especially Guild Wars 2!)