History. It was a way of letting people who don't have TVs not pay a license. There was also an extra charge for colour TVs.
Edit: Oh, the BBC isn't a government agency, it's operationally independent. But always becoming less so, as the board is mostly appointed by the government now.
Edit: Oh, the BBC isn't a government agency, it's operationally independent. But always becoming less so, as the board is mostly appointed by the government now.