The word may have fallen out of favor, but I think the concept of "good, but not required" is alive and well in almost all folk morality. It's troublesome for (non-divine-command) philosophical approaches because you have to justify the line between 'obligation' and 'supererogation' somehow. I suspect the concept might sort of map onto a contractarian approach by defining 'obligatory' as 'society should sanction you for not doing it' and 'supererogatory' as 'good but not obligatory', though that raises as many questions as it answers.