> Right, but why go with mobile at all? I get the laptops.

Pair a power-efficient mobile chip with a mini-desktop form factor and a good (i.e. probably overengineered, to some extent) cooling solution, and it will give you a kind of sustained performance and reliability over time that you just aren't going to get from the average consumer/enthusiast desktop chip. Great for workstation-like use cases that still don't quite need the raw performance and official support you'd get from a real, honest-to-goodness HEDT.