Ideally you'd use the most common name, in order to maximize the size of your anonymity set. So you could go with "John Appleseed" to get a "john" home directory and a "John's MacBook Pro" hostname. But in a small community, to local observers seeing your device on bluetooth screens, you might get weird looks ("there is no John here, why is this sus?") whereas "iPhone" looks like a default. And for non-local observers, your choice of a name like "John" suggests you're a westerner named John. So by just going with "iPhone" or "MacBook" you're in a smaller anonymity set but also not drawing attention to yourself in your local area, and not externally leaking information like your locale (although of course you could always leave a false trail by using a cyrillic hostname).
Some other leaky, seemingly private identifiers are SSH pubkeys (I always delete the comment trailer), which are sent to every server you SSH to and also published to places like GitHub, and WiFi SSIDs (which are visible to any application with access to the network stack, and unfortunately aren't entirely within your control - often a list of nearby SSIDs, combined with a mapping of SSID to geolocation, is enough to triangulate your location to within a meter, which is one of many reasons I disable WiFi in favor of ethernet whenever possible).
I do the same, mostly because I don't like my full name showing up in every subpath of the home directory. Sometimes this leaks far beyond your local computer, too, since many build tools include some path info in the compiled binaries. You can download apps from the app store, run `strings` on them, and find the username of the developer who built the binary.
When you want to add a sandboxed scripting language to any system, you might implement something like this. For example, a multi-tenant web proxy that lets users script functions for transforming requests, or a game that lets users script the user interface elements, would be two valid use cases.
It's probably something where the government wanted to stop this deal for a number of reasons, and "national security" was the easiest justification where they already had a legal apparatus in place for doing so.
It's in line with recent behavior toward e.g. Huawei and 5G. What's interesting here is that Softbank is a Japanese company, but reading the government announcement you'd think it may as well be Chinese. It would be interesting to see the UK explain to Japan, somewhat of an ally, why its company is a national security threat.