Obviously, either doing the thing is optimally moral, in which case you ought to do it, or it isn’t, in which case you should instead do the optimally moral thing.
That’s all downstream of an implicit definition of “what I am obliged to do” as “the optimally moral thing”. If what you are obliged to do is less demandingly, then there is space for the superogatory.
Such a system doesn’t prescribe which action from that set, but in order for it to contain supererogatory actions, it has to say that some are more “morally virtuous” to others, even in that narrowed set. These are not prescriptive moral claims, though. Even though you follow this moral system, a statement “X is more morally virtuous but not prescribed” coming from this moral system is not relevant to you. The system might as well say “X is more fribble”. You won’t care either way, unless the moral system also prescribes X, in which case X isn’t supererogatory.
That’s all downstream of an implicit definition of “what I am obliged to do” as “the optimally moral thing”. If what you are obliged to do is less demandingly, then there is space for the superogatory.
If I am not obliged to do something, then why ought I do it, exactly? If it’s morally optimal, then how could I justify not doing it?
Many systems of morality are built more like “do no harm” than “do the best possible good at all times”.
That is, you are morally obliged to choose actions from a particular set in some circumstances, but they do not prescribe which action from that set.
Such a system doesn’t prescribe which action from that set, but in order for it to contain supererogatory actions, it has to say that some are more “morally virtuous” to others, even in that narrowed set. These are not prescriptive moral claims, though. Even though you follow this moral system, a statement “X is more morally virtuous but not prescribed” coming from this moral system is not relevant to you. The system might as well say “X is more fribble”. You won’t care either way, unless the moral system also prescribes X, in which case X isn’t supererogatory.
There are things that at good to do, but not obligatory.