Please explain twist bones

I am not a professional 3D artist but I do a lot of 3D in my spare time, I have read countless articles, books, and tutorial videos and even did a google search on the subject matter but there appears to be no answer to what problem upper arm and upper leg twist bones solve. I sort of get the lower arm and calf twist bones which mimic the turn of the Radius and Ulna and the Tibula and Fibula but have no clue (and this is most likely my noobishness) as to what the upper arm and thigh twist bones are supposed to solve and how they should be weighted.

Someone please help this poor noob.

Anyone? Anyone have an answer???

Check the weights/bones of the mannequin mesh.

I am not certain, but I believe verts with some weights on those bones can deform more realistically…