InSAR is mostly used for DEM generation, so to convert phase differences between two images to relative heights. To get absolute heights a reference DEM is required.
The D in DInSAR stands for "Differential", that means it is used to observe phase changes between two images in a given time. It is used to monitor subsidence/uplift or lateral deformation. To be indipendent from topograpy, the topographic phase is simulated using a reference DEM and then removed from the interferogram.
This might be interesting to you, especially chapter 6: