At the time, many felt that the functionality of the delivered software was the priority and would often seek to lock the scope of the project.  We allowed for this by having some degree of variability in the budget and the schedule.

At the time, many project managers would work to the PMI or ISO 9000 guidelines on the definition of quality. Both of these had a reasonably straightforward quality definition requiring the scope of the project to be delivered in a fully functional form. 

To meet the scope expectation, we had to estimate, to a fair degree of precision, how long it would take us. In this way, we would be able to determine the length of time needed and the number of people required for the team.

And it was at the estimate stage that we often set ourselves up to fail.