Hi Rachel.
Yes you are right, there is some medicinal use of the roots as well, they just aren’t used as often it would seem. I use mullein root on occasion in mixes for antispasmodic pain relieve especially in the back and jaws. Here’s a nice overview: https://clearpathherbals.com/medicinal-herbs/mullein-uses-flower-leaf-root/
With warmth,
Benna