Dem and elevation calculation

When do INSAR backgeocoding or geocoding, DEM is needed. SRTM or external dem can be used. I check the source code and notice that the elevation is calculated by dem+EGM96 by default.
In geocoding, there is a option about adding egm or not. But in backgeocoding, there is no option. I don`t know much about this, do we have to add egm when calculating elevation?