In this case it might be a 'documentation as code'thing, being able to see at a glance if something is a language primitive or a potentially very different implementation could have value.
However I'm not super familiar with Rust, so I couldn't speak to that why.
In C++ it can matter for correctness because primitives are uninitialized by default. But other types might be too and the standard library uses under_scores for things that are initialized on construction, so it's not a great example of this distinction.
Why do you care in other languages? In Rust for example I'm a little fuzzy on why I care if something is considered a primitive.