Apple has done it, so Google will do it. That seems to be the most logical reason behind why Google is set to start designing its own chipsets. The report comes from this article which claims that in order to add more features and have innovative stuff happening on their devices without depending upon third party to enable them. So far, all the Google Nexus phones have got a chipset on them that have been manufactured and designed by third party manufacturers like Qualcomm. This does not give Google the freedom always to experiment and add things like Apple can do, where they design the chips themselves while the manufacturing is done by the likes of Samsung.
Having the ability to design the chipset in-house has allowed Apple in the past to bring health tracking thanks to a co-processor and play around with the position of that bringing it right inside the main chipset allowing more space to be allocated to more components like bigger battery size.
Google has already flirted with the idea of making their own chipsets when they were mulling over coming up with an enterprise focussed device in the past. There is no solid confirmation on who are the chipset manufacturers will Google approach for the manufacturing, but you would think the likes of Qualcomm, Mediatek and Samsung would have been a part of the discussions. Quite possible that the Nexus phone next year, could well come with a chipset that Google themselves have designed.