The point is to have the system automatically unlock without the need for a boot password. This provides decent security if secure boot is enabled, but requires very little from the user. It isn’t a stopper for high threats, but a simple theft will mean the data is safe. It also ensures that if the drive is separated from the host machine, it is useless without a copy of they key. It doesn’t stop all threats, but stops a lot of them, and all of the most common.