The hdcbb-x is a very useful stitch to use in order to create a soft 3D effect.
Sometimes several special stitches are made in a row. When single crochet are worked into the top loops of the special stitches in the next row, a gap is left in the back (on the WS) of the special stitches (as we need to skip 1 stitch behind each special stitch in order to avoid unnecessary increases)
In case a dcbb is worked into the skipped stitches, the special stitches in front of each dcbb worked will need to be skipped. This will create a sharp ridge and leave a gap on the RS of the work.
By using a hdcbb-x we can connect the special stitches in the front with the gap-filling stitches in the back while also creating a special 3D effect.