Political candidates in the United States have to have huge amounts of resources in order to compete with other political candidates. This can set forth an unfair advantage for politicians who have a lot of money, compared to those who don't. We've all probably noticed that it is the most wealthy of people who often run for office, especially higher offices. By giving free airtime to all candidates, we would help to level the playing field, even just a little bit, potentially encouraging people from all walks of life to campaign.
Nearly all the other democracies of the world provide free air time to political candidates. The only two overseas elections I've covered were the Angolan election in 1992, and the South African election in 1994. Both were first-time democratic events. In both, the candidates had free air time. The first election led to renewed civil war, the second led to an end to 300 years of oppression. So based on this tiny sample, the record is mixed. (It's also worth noting that only a fraction of the residents of those countries have televisions.) Free air time isn't a panacea, but it's a step in the right direction.