A small lock around a chain just doesn't seem strong enough. The chain can be easily broken and the thief can just coast away with the bike using it much like a push-scooter.

If your bike has a rear disc it's really easy to put a padlock through the disc to prevent it from being ridden away. And that rear disc is not easily broken so it's a bit safer.

My old eBike had one built-in and it worked because the ebike was just too heavy to be carried away.