Why was he wandering the streets?
Griffin was wandering the streets because he had become a homeless wanderer without clothes or money. This happened after he set fire to his landlord's house in revenge for trying to eject him.
To escape from the scene of the fire without being seen, Griffin had to remove his clothes to become completely invisible. As a result, he was left wandering the cold London streets in mid-winter with nothing, completely invisible until he stepped in mud and left footprints.
Explanation
The textbook explains that Griffin was a lawless person. When his landlord tried to eject him, Griffin set fire to the house in revenge. To escape the crime scene unseen, he had to remove his clothes to use his invisibility, which left him without clothes, money, or a home, forcing him to wander the streets.