1) It allows them to actually work with hardware and cross pollinate knowledge to their consumer hardware businesses. And I’m not talking only about the hardware itself but all the institutional knowledge required to negotiate with vendors, outsource circuit design and procure parts.
2) They can create one device that is multi-purpose like this one and control every low level detail so it can be used even for security critical tasks.
3) They can test the devices against their own infrastructure and collect low level data that is useful for developing video/voice services and encrypted services in real case scenarios.
In general it just makes sense. Facebook’s market cap is 6X larger than Cisco and it employs 50k people. At that scale and for a company like FB, this doesn’t sound crazy.