Amazon buys the network Wifi company Eero, to strengthen its smarthome offerings. Because more smart devices rely on Wi-Fi, Amazon needs a powerful network connection, and Eero is the solution.
If nothing else, CES 2019 showed that Wi-Fi smart devices come in full force. Unfortunately, current Wi-Fi standards are less powerful and exceed lesser distances on their own than ZigBee or Z-Wave network standards, and this is a problem for Amazon's smart home ambitions. Most of Amazon's current smart home offers (from Call to Echo to their smart outlets) are Wi-Fi devices.
While Google already has a networking system that can help with this problem, if Amazon wants to offer wireless Wi-Fi is needed to either build a new system from scratch or buy a robust existing system. It chose the latter and chose Eero for the latest acquisition.
Amazon and Eero say they can work together for a better smarthome experience, as Dave Limp, SVP of Amazon Devices and Services, explains in a press release:  We have a common vision that the smart home experience can be even easier , and we are determined to continue to invent for customers' sake.
Amazon's purchase of Ring seems to have gone smoothly, so if it is something that Eero should continue to be a viable product after purchase, as opposed to being shut down and folded into another existing product.