I would guess that probably 75% of mac users have never heard of VersionTracker, MacUpdate, or any such sites and have probably never voluntarily sought out & downloaded as much as a single application on their computer in their entire lives. They may have loaded something if prompted as a result of an error message--"to view this content you need the latest version of flash, click here to get the latest version--but probably have never loaded anything else on to their computer.
If you don't believe me, just go to your local Apple store and casually walk around and eavesdrop on the conversations between the employees there and people buying computers. Really. Just eavesdrop a bit. And tell me how many of those people are taking their macs home, thinking to themselves "Hmm, I'd really like a program that does [X], I'll to go MacUpdate and do a quick search and see what I can find, download a demo of it, try it out a bit, then go to the developer's website and buy the full copy if I like it." It ain't happening. Just eavesdrop a bit and you'll see what I mean.
Now if you give them a simple 1-click way to "discover" all of these other apps, decide on something, buy it, and install it? All with 1-click, no need to enter any information, no need to worry about how to install it, nothing, just 1-click? Suddenly even a total n00bie mac user can do that. And many, many will.