The lifter lock is for keeping the chamber empty with ammo in the mag tube. Crossing fences, travelling from one place to another, etc. Keeps it "unloaded" with the ability to quickly load.
It's good to know the Nordic Components nut works. They list it in the drop down box on their website, but it doesn't display in the text below after it is chosen. I emailed Nordic about it for clarification and haven't heard back.