Based on my own experiences, here are a few reasons that I can think of:
+ Has a setup that involves 3 or more screens
+ Needs to use 2 or more hard drives
+ Likes the idea of having a video card that can be easily upgraded
+ Likes the idea of having a processor that can be easily upgraded
+ Needs to have two or more video cards
+ Needs to have two or more optical media drives
My laptop can use 2 or more hard drives.
Why does a developer need to upgrade the video card in their laptop? What is it they're doing? There are a lot of developers, and a fairly small set of them care about fast graphics.
Why does a developer need to upgrade their processor more than every few years? What is it they're doing? For the relatively rare developer that still has long compiles to deal with, why doesn't their company have a build farm?
Why does a developer need two or more video cards? What is it they're doing? Again, most developers are not in gaming or other high-end graphics work.
My laptop can use two or more optical media drives.
Laptops are not a little black box with zero expandability. Ports exist for a reason.
Because while development might be a use, or even the primary use, of a machine, it doesn't mean it is the only use.
Some people will want to watch videos, some play games, some just want the newest hardware to feel good about themselves, there's a bunch of reasons.
Anyway, my laptop is my one and only front-and-center machine, used for development, gaming, videos (why you brought that up, I have no idea, since it takes hardly any power at all), etc., and I have no performance problems.
But there are plenty of developers (on HN in particular) for home "their machine" isn't just something that is in their office for work use, but is what they have for personal use as well (maybe they're freelance, maybe they work for their own startup, whatever).
I've seen you reference this multiple times now. Based on experience as a consultant, I think you are giving most companies (or at least their IT dept) way too much credit.
Most companies have no need of build farms, the code the developers build simply isn't that big, and definitely doesn't need the latest hardware to be done quickly.
I would definitely expect companies with massive projects to have build farms, even if the developers have to take matters into their own hands.