At that time the waterfall model method was one solution to the problem of software quality. Other methods hadn't yet been developed (or at least not become well known yet), and waterfall was in use by the industry (not everywhere though - re sibling comment). The customer I mentioned was huge and lots of companies in lots of European countries, and overseas too had development contracts which demanded this process to be used. And the results were actually good, even though the process was heavy. That didn't mean that it couldn't be improved on, and that did happen later, with more agile-like (though not "Agile") methods modifying the approach.