Why are there barbed wire in Hong Kong and Shenzhen?

Hong Kong and Shenzhen have barbed wire to prevent illegal entry. Across the sea, the road with protective nets is a high incidence of smuggling and smuggling. Across the Shenzhen River, the distance is 20 meters. Shenzhen and Hong Kong are directly separated by the border, with Shenzhen River and barbed wire. Border guards are stationed on both sides to manage. Mainlanders need travel documents to enter Hong Kong. People without travel documents are illegal immigrants. You can enter Hong Kong in 15 minutes by car, which is very close to the city. It's an hour's drive.