They aren’t really designed to stop people from breaking in but rather to stop intrusive people that you talk to first before deciding to not let them in. With the chain, you can open the door a bit without allowing the person outside to force himself in without too much force, e.g. by blocking the door with your foot as the door can only be opened fully after closing it.
Advanced versions exist where if you put strain on the chain (mostly trying to push the door open from the outside) an alarm goes off.
Door chains are a funny concept to me. They look flimsy and not really suitable to keep anyone out.
They aren’t really designed to stop people from breaking in but rather to stop intrusive people that you talk to first before deciding to not let them in. With the chain, you can open the door a bit without allowing the person outside to force himself in without too much force, e.g. by blocking the door with your foot as the door can only be opened fully after closing it.
Advanced versions exist where if you put strain on the chain (mostly trying to push the door open from the outside) an alarm goes off.