It means the station does not have to pander to advertisers, meaning the programmes can be more honest. Programmes like Top Gear UK, where they actually can honestly rip into the cars and criticise them properly would be impossible on private television. You can rip the godawful corvette only once before GM drops its sponsorship and advertising on the station.
Because the money a private station gets depends on the number of viewers, a public station does not have to rely on high ratings, and thus can have more quality programmes, instead of programmes that bring high ratings. More educational programmes instead of Beavis and Butthead.