Re-reading Thief of Time for the umpteenth time and as usual picking up new things.
This time, a little scene stood out that may once and for all explain who Susan really is and why she is the way she is.
When she meets Wen inside the world of Time, he describes her as "the personification of helpfulness."
I hadn't paid attention to his before, but this time it made the penny drop. Just as Death, Pestilence, Famine, Kaos, War, the Hogfather, the Tooth Fairy, the God of Hangovers and other "mythical creatures" are the anthropomorphic embodiments of human concepts, Susan has "transformed" from a pure human to become the Goddess of Helpfulness. Her metamorphosis is different than the others, since she started out as human and was born of humans, but as her character evolves through the books it becomes clear that her purpose is to "help" others solve problems, whether it's helping Death "save" the Hogfather (and the "belief" of children), helping Lobsang fulfill his destiny as the god of Time, helping scared children overcome their fears of bogeymen, or helping students achieve their full potential. The "helpfulness" thing doesn't quite totally gel in Soul Music, probably because PTerry hadn't totally figured out Susan's trajectory at this point, and it's not evident whether Death fully understands her ascendancy to this role (he sees her most as his assistant than anything else), but ultimately her supernatural powers "evolve" not because of her status as Death's granddaughter but because they're shaped by the same magical forces that create the Discworld's other "gods and goddesess."
Sorry if this has been discussed before.