The SAR sensor makes observation in Line of Sight (LOS) direction (this is the SAR geometry) as shown in figure below:
Fig 1: LOS direction
Hence, whatever measurements we do, the results are going to be in LOS direction. In order to derive subsidence in vertical direction (we assume that the subsidence occurred vertically and not east-west), we need to do some trigonometry. So, in the equation given in the tutorial you have read, the cos (theta) is added to the equation in order to get the results in vertical displacement and not in LOS. The displacement values though, are still relative (but in vertical direction not in LOS), which means we still need to subtract a value of zero dsiplacement to get absolute values.