The gdal_translate utility

The official documentation for the gdal_translate utility is ​http://www.gdal.org/gdal_translate.html.

when to use -a_ullr - Assign/override the georeferenced bounds of the output file.

Use the -a_ullr option to add coordinates and pixel size to your raster. Using -a_srs alone only adds information about coordinate system without modifying the coordinates themselves ( i.e. upper-left coordinates, pixel size and so on). So if the source image doesn't provide this information to gdal they need to be added by hand.