The incidence angle of SENTINEL-1 wide swath mode varies between near and far range (15-49 degrees). This is why we need to set a reference incidence angle.
In order to correct for the wide incidence angle, we need to do incidence angle normalization. Most of the authors, suggest that the incidence angle should be normalized between 30 and 35 degrees. Take a look at the paper below:
Also, in SNAP you can perform radiometric flattening that corrects the radiometric distortions.
This tool in SNAP is based on the following article posted by ABraun.