There is an amusing post on VentureBlog into the whys of multiple stage investing:
Sillywood, Part 3: All The Money is in Sequels: “There is a reason both Silicon Valley and Hollywood are mostly about sequels.
One additional point to is the timing associated with those “sequel” investments. A few months of continued development can drastically reduce the risk associated with an investment, even though the valuation of the company is essentially the same. Which, of course greatly increases a company’s attractiveness to a VC.