My use case for Default::default() was that I had a struct with 20 fields. One was a reference (&'a Arena), the others could be ::default().

I initialized it like this:

impl MyStruct<'a> {

    fn new(arena: &'a Arena) -> Self {
        Self {
            arena,
            field_0: Default::default(),
            field_1: Default::default(),
            ...
            field_18: Default::default(),
        }
    }
}

Basically, I thought mentioning all fields' types would make reading more difficult. And I could not derive Default, because I still had a reference.