It could be that the elevators are out of order and he's in a hurry to get out of the building, so he chooses to climb down 15 stories. There might be an important meeting or appointment he doesn't want to be late for. Also, perhaps he just wants some adventure or a physical challenge, although climbing down 15 stories is quite a risky adventure. He might be a thrill - seeker who likes to do extreme things.
It could be that he is in an emergency situation. For example, there was a fire in the building and the stairs were blocked, so he had no choice but to climb the outside of the building. And he might have some basic climbing knowledge and enough courage to attempt this dangerous climb of 15 stories.
If he was climbing for a legitimate reason, like a rescue operation or if he had prior permission from the building owner, then it could be legal. But just randomly climbing 15 stories of a building in Philadelphia without any authorization is likely against the law.
It could be a dare or a part of some extreme fitness challenge he set for himself. Some people like to push their physical limits in unique ways, and climbing 15 stories is no easy feat. It might be his way of proving his strength and endurance to himself or others.