default-toplevel-value and set-default-toplevel-value

[Eli Zaretskii]

These functions were evidently introduced in Emacs 24.4, but they are
not documented, neither in NEWS nor in the ELisp manual.  The doc
strings are minimal, and leave me confused about how these are
different from default-value and set-default.  Is this just about a
possible let-binding of a localized variable, or is there something
else involved?

And why did the commit that added these functions also made changes in
Custom functions?

TIA