What is the wisdom behind prohibiting a woman from fasting supererogatory fasts except with her husband's permission, even though it is an act of worship, and there is no obedience to a creature in disobedience to the Creator?

It is not permissible for a woman to observe a voluntary fast while her husband is present unless she has his permission, because his right is obligatory and takes precedence over voluntary good deeds. The Shafii scholars have explicitly stated the prohibition of her observing a voluntary fast without his permission, as he has the right to enjoy her at all times. Her not observing a supererogatory fast is not a sin; rather, the sin is her not observing the Ramadan fast, and for that reason, she observes it without her husband's permission.