Locked is good if you don't intend to share a scene with someone who wouldn't have bought the object. If you work with someone else on a scene, and that person doesn't have the object, he will only see a grey dummy box instead of the object.

Non locked doesn't mean there is another 3d file format on offer, it just means the object will be seen by someone you send your scene to. And sometimes, but not always, non locked offers other 3d file formats, readable by other apps.

If you intend to use the object only for yourself, on your own Vue licences, just buy locked content. It's cheaper, and it suits your needs.