Let's look at a possible scenario.
The text has an unnumbered first chapter, where equations are identified by letters, while the body of the book has equations numbered like (chapter.equation).
% this and 'oneside' is just for making small pictures
Some text followed by an equation
and here we use an equivalent formulation of an equation in the introduction
Some other text
\ref in the recalled equation allows to make this independent of the actual number used in the introduction.
The author might have not used
\renewcommand\theequation and have assigned “A” manually with
\tag; but the result would be the same
% in the introduction
% in the body