A two-stage optimization is useful for the following applications:
An optimization by means of a grid structure is to follow. See tutorial ...
A very fine high-resolution structure is desired. Then it is advisable to first perform an optimization up to a reduced structure (the optimization must not be finished). Then a second optimization with a start design is performed, which starts directly at a higher resolution level and receives more iterations in these levels to form the fine structure.
You want to change some design details of final structure. You can remove struts / add rough struts with any external software and use this new .stl file as input / start space for the optimization. The algorithm will take care of an optimal change of the shape.