In a world where truth is unknowable and men's hearts are filled with either greed or a primitive darkness that threatens to overwhelm them, Marlow seems to find comfort only in work. Marlow notes that he escaped the jungle's influence not because he had principles or high ideals, but because he had a job to do that kept him busy.
Work is perhaps the only thing in Heart of Darkness that Marlow views in an entirely positive light. In fact, more than once Marlow will refer to work or items that are associated with work (like rivets) as "real," while the rest of the jungle and the men in it are "unreal." Work is like a religion to him, a source of support to which he can cling in order to keep his humanity. This explains why he is so horrified when he sees laziness, poor work, or machines left out to rust. When other men cease to do honest work, Marlow knows they have sunk either into the heart of darkness or the hollow greed of civilization.