Well, the system-wide "internet available?" variable isn't completely useless from a user perspective.
If youtube.com happens to be down for whatever reason, there isn't much the user can do about it except wait, so there isn't much for the YouTube to do besides display a standard error message and call it a day.
But in the more common case where the user's internet connection is unavailable or misconfigured, there are troubleshooting steps the user can take (make sure it's plugged in, try turning the router off and on again, check network settings, etc...).
I suspect the design was to help users get their TVs connected to the internet by providing more useful diagnostics when they are offline. To do that, you need some kind of system-level check for connectivity. Now, there are certainly better ways they could have implemented the feature, ways that didn't involve a single point of failure where a fire at one building means that millions of TVs can't access Netflix or YouTube for no good reason, but the intent wasn't completely meaningless.
If youtube.com happens to be down for whatever reason, there isn't much the user can do about it except wait, so there isn't much for the YouTube to do besides display a standard error message and call it a day.
But in the more common case where the user's internet connection is unavailable or misconfigured, there are troubleshooting steps the user can take (make sure it's plugged in, try turning the router off and on again, check network settings, etc...).
I suspect the design was to help users get their TVs connected to the internet by providing more useful diagnostics when they are offline. To do that, you need some kind of system-level check for connectivity. Now, there are certainly better ways they could have implemented the feature, ways that didn't involve a single point of failure where a fire at one building means that millions of TVs can't access Netflix or YouTube for no good reason, but the intent wasn't completely meaningless.