From a Forbes article: "AWS sees networking as the critical path for cloud but also realizes that the relative cost of networking increases as server and storage prices drop rapidly. Networking is, in Hamilton’s words, “frozen in time” as the vertically integrated ecosystems generate “indefensible profit margins”. AWS’ solution is to build a custom networking hardware and protocol stack and to own their own private long haul links."
from this Forbes article: http://www.forbes.com/sites/benkepes/2014/11/25/scale-beyond...
So Amazon is really really trying to reduce the cost of Networking.
From the extremetech article: "Analysts have pointed to server networking chips as Annapurna Labs’ main focus. These chips could be used to move data more efficiently and save power in data centers."
I think it's pretty clear why they are making this aquisition...