If one assumes that journalism is the product of a free press, then journalism is important to maintain an informed electorate. There are no secret police, courts of star chamber, etc. Journalism is the most effective way to hold government and elected officials accountable to the electorate. It also prevents abuses by powerful people even if they are not part of government structure. I shudder to think the system of government one would have if there were no journalism to hold people accountable.
I assume that you are asking how journalism helps a society in terms of politics.
If so, journalism is so important to political freedom that it was included in the First Amendment to the Constitution. This is because journalism is vital to the functioning of a democracy in at least two ways.
First, journalism is necessary so that the public will know about the issues that face the country. If it were not for journalism, the general public would know much less about things like the actions of companies that benefited from the bailout after the financial crash of 2008. We would not know nearly as much about political movements like the Tea Party. In short, we would be ignorant about important things that are going on in our country. As a counterexample, Chinese journalists were not allowed to report that their President said human rights are important during his dinner with Pres. Obama. This denies them the opportunity to know something important about what their leaders have said.
Second, journalism acts as a check on government leaders. Because journalism exists, public leaders must be more careful not to do things that would make their constituents angry. It is because of journalists, for example, that the Watergate scandal was unearthed. This sort of "watchdog" role is vital in helping us prevent corruption and other negative actions by our leaders.