The BBC helps keep program standards up
As the BBC is free to watch (and listen to) and does not carry outside advertising, it puts some limits on how bad a pay-TV channel can be. If the program quality is too bad or the advertisments too intrusive then people will watch the BBC instead. The presence of the BBC stops many channels from being as bad as the US ones (40 minutes of advertising in a 60 minute program).
The BBC also carries good quality regional programs that would probably never be produced in a pay-TV system.