1) In the US, its everywhere. Usually, the more urban the location, the more channels you can get.
2) It used to be the only way to get TV. And it costs a tiny fraction of any other type of content provider. So you could say its essential to low income people or people who think its immoral to support comcast.
Everywhere is a bit of an overstatement. Most, if not all, urban and suburban locations with reception rapidly falling off in exurban and, especially, rural locations (i.e. the vast bulk of the land area of the US). Obviously on a population basis, the OTA situation is a lot better but there's still a pretty rapid falloff as you get 30 miles or so away from urban centers with TV stations.
Over the air TV is used by the Emergency Broadcast System to alert residents of natural disaters and inclement weather. For low-income families, this is essential. The town I'm from was hit by a tornado in 2005. The NOAA weather radio transmitter did not work properly, leaving only OTA broadcasts to alert residents of dangerous areas like mobile homes.
1) Is over-the-air broadcast TV widely used anywhere? Very few even know about it where I live, but maybe there are places where it's more common.
2) Is it essential to any group of people?
3) Could we get more beneift from that spectrum by repurposing it for something else?