I'm not sure if you're being obtuse; this is basic organizational politics.

Pretend that Facebook's hitherto web-centric engineering team is external to Facebook -- they're a consultancy. Now imagine Facebook comes to them and says: We want a mobile application. Who benefits if they manage to convince Facebook to pay for them to produce a web application?

Let's reel it back to reality. How do people inside an organization benefit if they expand their area of purview, responsibility, and increase their value to the organization? Likewise, what power dynamics come into play when a new, potentially distinct engineering team garners additional purview, responsibility, outside of your domain, skills, or control?