One of the ideas is that brand new sites are less resistant to spammy links than older, established sites. Blasting a brand new domain with really shitty backlinks is a good way to get it banned fast or permanently penalized - I have done this more times than I can count.
When you set up web 2.0s, splogs on established free blog hosts etc and then link to your money site from them, your money site starts to inherit some of the editorial trust that those web 2.0 domains have. Those web 2.0s are also more resistant to spammy links (because they are on a trusted, aged domain with plenty of existing backlinks), so you can start generating link juice by pointing shitty links at your web 2.0s and thus funneling it to your money site. The web 2.0s won't get penalized as easily and so they will carry the link juice through to your money site.
Once your money site has aged a bit, gotten some indirect link juice and trust from the web 2.0s and is firmly woven into the fabric of the internet, then you can start pumping some more spammy links directly at your money site.
Of course, you can also start pumping direct links to your money site from day one. It works well in some situations - it just depends on your exact linking strategy. If you can get some tasty authority links right from the start, that immunizes your money site somewhat against the negative effects of spammy links. I recommend testing both ways and seeing what works for you.
A good way to look at it is that you want to mirror the linking pattern of a site that goes viral - ie. quickly acquires a shit ton of links. For example, first a small proportion of the net discovers this brand new thing and raves about it. It gets picked up by the social networks, social bookmarking sites etc and has a huge rush of incoming links and traffic from authority sites. Then all the second and third tier blogs etc start posting about it in the coming weeks, then the scrapers start picking those links up, and links to the site are eventually spread far and wide through the bowels of the internet.
What would the rate of link acquisition be? What sites would pick up this new and wonderful site? What kind of anchor text would be used for each different type of site that links to said new site? How would the backlink profile of a naturally viral, high quality piece of fresh content look compared to someone who just started link spamming their new crap whatever way they could? All questions for you to consider...