It's a stupid way to get into a specific network, but it's a pretty convenient way to get access to everybody's networks. Sure, it would be hard to hack Google, but the prize for doing so would be pretty nice. And if the systems are impregnable, there are always ways in that relies on the human factor. Humans can be bribed and/or threatened.

Or if you're law enforcement, the knowledge of the existence of the resource is enough.