Not just when they don't need to modify the argument, when they don't need to consume it. The way these parameters are written takes ownership of x, it's gone, not just modified.

For a non-trivial object (which apparently x is), you would always prefer to write &x (immutable reference to x) or at least &mut x (mutable reference, not applicable here because x is not defined as a mutable thing).

So apparently these functions intend to consume x. That's not common but there are a reasonable number of Rust APIs which consume larger things on purpose.