I'm personally making worlds simply to see the various reasons behind making the slabs.
The main themes I'm seeing are "..after pondering the ineffable subtleties of <sphere>" and that either demons have linked spheres (Usually when the deity's sphere was negative) or unrelated spheres. Can't confirm an overwhelming bias towards certain spheres (Other than perhaps
So far out of twenty tries I've had..
"..after pondering the ineffable subtleties of youth." (From a goddess of youth, children, dance and festivals - the demon was associated with youths and suicides.) (Once.)
"..to provide opportunities for bravery to rise." (From a goddess of courage - the demon, curiously, was only associated with theft and food.) (Twice.)
"..to provide opportunities for acts of valor to be performed." (War, Valor, Fortresses - Disease) (Twice.)
"..after pondering the ineffable subtleties of jealousy." (Jealousy - Jealousy) (Once.)
"..after pondering the ineffable subtleties of treachery." (Treachery, Trickery - Treachery, Lies) (Once.)
"..that great fortresses might be raised and tested in siege." (Jewels, Fortresses, War - Death, Disease, Deformity, Blight) (Seven times.)
"..that it might bathe in misery forever." (Torture, Misery - Misery, Luck, Gambling) (Once.)
"..that more might die." (Victory, War, Death - Suicide, Gambling, Luck, Death) (Once.)
"..in order that acts of torture be perpetrated." (Torture, Misery - Order, Blight) (Once.)
"..pondering the ineffable subtleties of war." (Victory, War, Fortresses - War, Trickery, Lies, Fortresses, Chaos) (Once.)
"..because it was destined." (Fate - Disease) (Once.)
"..after pondering the ineffable subtleties of blight." (Thunder, Blight - Murder, Death, Blight) (Once.)