There are a few reasons I want this outside of file syncing.
1) It somewhat feels like it could be less wasteful. It may only be on the appearance level, but I end up with a new phone about every 2 years, a new computer about every 3, a new monitor...much less often. If my phone was a smart client for these other input devices, I feel like I would be upgrading my phone about the same, but wouldn't be upgrading a laptop as often.
2) It "feels" like the same device. Though I have file sharing, I don't have desktop sharing. The file I stuck on my desktop on my laptop isn't in the same space on my phone. It feels like they are synced, rather than just being my workspace.
3) I have to duplicate my apps on each device, and that is not always possible, so I end up with a comparable app, or having to switch devices. This is becoming less common, but still, do I need to have multiple versions of the same software on each different device and different OS?
4) Notifications everywhere. When I'm being summoned on multiple messaging apps, I'm getting notifications on at least 3 devices! Yes, I could turn notifications off, but obviously, I need some notifications. If I had only one device, acting as the smart hub to other input devices, in theory, it would know which device I'm active on, and just ring that.
At the same time, we work a bunch with graphics, so I don't think I'm getting decent developer capable graphics card in a mobile device anytime soon, but I think other users could move to this sort of paradigm fairly easily.