Its like a firewall at the edge of your network: doesn't really protect you as any attacker that can get to the other side has free reign. You need defense in depth.
By the way, a piece of pedantry apropos a recent HN article: "...the Internet will devolve into regional internets." I.e. there is one Internet that connects to essentially everything; regional networks can practice internet working but aren't the proper noun "Internet"
I typed that on my phone and it auto"corrected" to add a space in internetworking. I'll leave the error in place and just post this comment instead even though I still have time to edit it.
By the way, a piece of pedantry apropos a recent HN article: "...the Internet will devolve into regional internets." I.e. there is one Internet that connects to essentially everything; regional networks can practice internet working but aren't the proper noun "Internet"