As per the plato's allegory of cave, one wise man managed to escape the cave with lot of hardship. He climbs up towards the light and once he realizes that there is a reality beyond the cave Plato argues that it is his moral responsibility to go back and save the those who are trapped.
I think batman so far believed that his lie about Dent's death had done good to the City and his city did not need him any more. All was well with the Gotham city. This is the imagined truth aka the shadows on the walls of the cave.
Batman's rise from lazaris pit is symbolic in the sense that with lot of hardship and pain batman realizes that there is truth beyond his knowledge. Gotham needs hope in the form of the same symbol that he had created 8 years ago.
Just like the free man in the Plato's allegory Batman could have just escaped because he did not owe anything to Gothamites. (Catwoman did suggest that). But he considers his moral responsibility to go back to Gotham with hope.