Part of it seems to be that for copyright they want to own the physical books that they have scans of. Instead of destroying the books in the process like is common, they are taking care to preserve the books.
Agreed, that's clearly part of it, but the overall thrust of the article seems to be 'if we don't do this physical books will disappear', which seems to be duplicating the work of national libraries.
It's a pity they seem to gloss over that, since the duplication of that effort does need some justification - perhaps the national libraries don't cover enough foreign books? Or they don't trust the government? Also it's a bit misleading, acting as if it doesn't already happen.
It is partly a matter of redundancy. Unthinkable as it might be, libraries do burn. They do have budget cuts. They get burglarized. Their archivists make mistakes. Better three universal libraries than two. Better four than three. When we're sure we have too many we'll deal with that; they are far easier to destroy than create.
It is true that things might be more efficient if everyone pooled resources and, say, kept track of the fact that we have ten distributed copies of Book X in the world but only three of Y, so we should be looking out for more copies of Y. Which brings us to a second reason to start a new public archive: as a pilot project to explore different ways of running public archives. Modern infotech is evolving fast; presumably there is lots of room to experiment with different strategies, and the national libraries have only so much budget for research.
Redundancy is why so much information survived from ancient times. When Rome collapsed, scholars elsewhere hung on to its knowledge and continued building on it while Europe pulled itself back together.
On the broad scale of history, neither national governments, nor their commitments to historical preservation, are something you can rely on.
I do not mean this as a modern-day political snark at any particular government, I mean this as a historical comment.
Of course there's no guarantee your archive will survive indefinitely either, but if we treasure them and have several of them the odds go up substantially.
