The baboon or the crow both have their obvious negative qualities. However these do not prevent them from shying away from some negative traits such as eating something that has died on its own. Crows are particularly interesting in that if they did not see the animal being killed they will refrain from eating it. From this observation our elders saw the lesson that regardless of all the negative traits, there is still hope for positive traits in everyone.
The proverb is used to reflect on the possibilities of virtue in every individual. It can be used to challenge or address a habitual offender or sinner to show some positive qualities by insisting that everyone must have some minimum standards or virtuous qualities.