UML Designer provides tools that support an evolutionary approach to modeling. Traditional modeling techniques have typically assumed a "waterfall" approach--moving forwards from requirements to implementation--but this is not generally the preferred approach for OO development, which tends to be iterative. In an OO project, it is equally likely that you will need to start with an existing implementation or prototype and use modeling to reverse-engineer the analysis for documentation purposes.
UML Designer is equally suited to either the "forward" or "backward" approach, or (most typically) a mix of the two.