The closest thing I can come up with is that the're modeling after companies like this:

http://www.textron.com/

Textron owns Bell Helicoptor, and EZ:GO Golf Cart company, among ~12 others, but the stock is all Textron stock.

I guess its kind of like internal diversification. I realize Google is a decently unique company, so I'm not sure exactly why they did what they did, but the model they jumped to seems common enough.

It gives the market more financial transparency into how the core business is doing, w/o merging in the effects of these other projects. More transparency means more accurate pricing, and thus a more reliable investment

I assume it signals that they will be investing more heavily in the non-Google portions of their umbrella. It also signals that they are going to keep Google as a very ad-focused company in their banner.

Probably because stock traders smell creative accounting here. Companies usually do that to minimize the taxes they pay, hence maximizing their shareholders’ profits.