In the Dark Knight Rises, after
Bane takes over Gotham City and traps most of the police force under the city,
there are several occasions where Blake draws the Batman sign in chalk on buildings and things. Why does he do that?
His basic goal was to give the people hope that Batman would return. Granted, people were not moving around much, but when they did, he hoped they would see this icon and know that people were still willing, able and were resisting the occupation of Gotham by Bane.
A curious aside, I notice the image drawn by Blake does have a strong resemblance to the chest icon of that famed first Robin, Dick Grayson who is now know by a name he has made his own, Nightwing. Perhaps a foreshadowing, a sign of things to come? We know that Nolan has vowed never to return, so it may have been a nod to comic fans of a possible future we may never see in the Nolanverse. Replace the bat ears with a bird's head. Maybe it's just a coincidence, maybe not.
he put the symbol on the buildings to mark the cargo trucks they where following so they knew the route so they could find the bomb and help.