There’s no situation for which “disconnect internet” could be the answer.
It's oddly enough the reverse.
From what it looks like, the idea is to try and make sure that the country's IT infruatructure and its essential public services - email, web, search, etc. - remain functional, should it be ever cut off from the Internet for whatever reason. It's a doomsday thinking, it goes against the very essence of the Internet, but this is something aimed at asserting country's independence.
I'd prefer this would've not been needed, but given the current state of affairs I can't say I'm shoked they are doing this.
Without the context, purely infrastructure-wise, what kind of scenario does "Roskomnazor will inspect the traffic to block prohibited content and make sure traffic between Russian users stays inside the country" imply to you?
It's just that many don't have any say in this and can't do much about it, but most realize this now. This is clearly a concern for both security and control reasons, which are two sides of the same coin. Any control can be abused. For every Roskomnadzor there's the US governement denying SWIFT to Iran and forcing EU to deploy a new system to work with the latter. So to me this looks more about power tugging at a country level with a freebie of a simpler censorship thrown in. Not the other way around.
Or before first strike, and attack from sources outside the country. In which case, seeing Russia go dark would be like an indication of cyber "missle" launch, but with probably milliseconds of early warning.
Makes me wonder if nations have or will have systems to try to detect when another nation is going offline and try to go offline themselves before damage can be done. Perhaps the best attacks will be launched from inside borders (imagine if one nation had hidden missle silos inside another nation's borders).
Then imagine if actual network problems were misinterpreted as a cyber "launch," and caused one nation to "launch" in response. Similar to ICBMs, there would be a window in which counterattacks could be launched, but it would be very, very small in comparison.