Knowing one's own history provides a group with a sense of unity as it unites them to know how their past brought them together through their past hardships.
It brings a sense of unity as people can feel they share a history together and have something to relate to, sort of like a nationalistic feeling in a sense.
Therefore, knowing one's history provides a group with a sense of unity as it allows them to feel as though they have something to relate to and have a past that makes them them, in a sense.
Hope this helps!
