It is a convention established in Emacs around ?40 years ago, that such
> Could you or someone else explain why an open paren in column 0 can affect
> the parsing of syntax in cc-mode at all? After all, such parens have no
> special syntactic nor semantic meaning in C nor C++.
an open paren signalled the start of a defun. This sped Emacs up
enormously, since the alternative was searching back to the beginning of
the buffer to check whether the paren was at the top level.