In some ways, I think we need a way to file a defensive patent, especially for a small player. So if you are a small firm, you can patent something and ensure that a big fish can’t also patent it and run you out of business, but without obligation (and maybe without rights to) enforce it if someone else uses your idea.
I.e., a patent that protects you against a later filer, but that’s it.
In theory, just publishing the invention means it'll become prior art and prevent its patenting. Supposedly the US patent examiners will review places like https://www.priorartarchive.org/ before issuing a new patent.
Prior art that was public knowledge before the filing is still supposed to invalidate a patent on the same thing. First-to-file means if person A was first to invent but kept their prior art secret (no publishing or filing), and then person B filed, person A's secret prior art can't invalidate B's patent.
As far as I understand patents still have to be "novel", i.e. new. You can't patent something that is already known to the public, otherwise people could start patenting chairs and sandwhiches.
Patents are meant as a reward for making an invention, the deal being that you get a monopoly on this new invention for a while and in exchange you have to tell the public how to implement your invention. If you could patent known things that would defeat the whole point.
What changed is that until recently the US operated under a first-to-invent principle, so if two people invented the same thing and they both wanted a patent on it, the one who invented it first would get the patent. As you can imagine, proving when you invented something can be quite difficult at times, and the whole process can be messy. First-to-file just means that in such a situation, the first person to file wins, which puts the US rules in line with other countries and makes things much simpler.
I.e., a patent that protects you against a later filer, but that’s it.