What's the purpose of things like `default-truncate-lines'?

[David Kastrup <dak@gnu.org>]

I read

    default-truncate-lines is a variable defined in `src/buffer.c'.
    Its value is nil

    Documentation:
    Default value of `truncate-lines' for buffers that do not override it.
    This is the same as (default-value 'truncate-lines).

    [back]

If it is the same as (default-value 'truncate-lines), why do we have a
separately named variable?

Is this just a technical necessity for some reason?

I am asking because `default-truncate-lines' is documented as a
variable of its own in the Elisp manual, and I am not sure whether
that makes any sense.  Maybe it would be better to point out that
`truncate-lines' is automatically buffer-local and thus its default
should be set with `setq-default'.

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum