Ask HN: Why did AirBnB raise $100million?

So by now we've all seen the news that AirBnB has a valuation over a billion dollars, raising $100 million for a website connecting people who want to rent out their space and those that are looking for it.

OK, so they are running a market-place web-site. They hold no inventory, do no physical distribution, and if they ate their own dog food they would have very little overhead.

Now, I know they probably have (plans for) a large marketing team, a dedicated PR team, some engineers, and executive management. But just because they COULD raise $100 million, doesn't mean they SHOULD.

What are they using this money for? Is this considered a 'lean' round given their plans? Why hasn't the coverage of AirBnB gone into more depth of their business plan, or any sort of justification on this valuation and funding?

They may have a good run rate, revenue, etc, but I still can't see a clear reason a web-site would need to raise that much money in one go.

Is there anyone else who knows more behind this deal? With the recent IPOs, and Techcrunch already covering AirBnB-like verticals, I think understanding the drive behind the money (if any) is valuable for everyone in the industry to keep their head on straight...