The UN General Assembly is sometimes hailed as an amphitheatre for global dialogue - and sometimes derided as an ineffectual talking shop. The reality is somewhere in between.

Every September, the world's leaders gather in Manhattan to open a new session of the global body. More than 140 leaders are in New York this week and one by one each will be given the opportunity to address the Assembly and the world from behind the behind the UN's marble and gold podium.

Brazil speaks first in a tradition that runs back to 1947 and the US speaks second as the host nation. Each leader is in theory limited to 15 minutes but it is difficult to drag a head of state offstage and in 2009 Libya's Muammar Gaddafi delivered a 90-minute diatribe demanding investigations into the murder of John F Kennedy and Martin Luther King.

While the television cameras focus on the UN's main hall the real intrigue goes on in a maze of nearby corridors and hotels. There leaders meet privately to deal and trade and this year President Barack Obama and his aides will be assiduously working to secure commitments from allies in the battle against the Islamic state.

Celebrities mingle with the world leaders, drafted in as UN "goodwill ambassadors" to lend their star power to issues of the day. Emma Watson has already made a high-profile speech on women's equality while Leonardo DiCaprio has spoken on climate change .

When the well-known faces depart, and the television cameras go with them, the regular work of the GA begins.

United Nations Secretary General Ban Ki-moon, and UN Women Goodwill Ambassador Emma Watson (EPA)

The assembly acts as a sort of parliament of nations, with representatives from each country debating and voting on the issues before it. Its size makes it unwieldy and every country is represented irrespective of its record on human rights or democracy. This year, the Assembly is chaired by a Ugandan MP who has championed his country's much-criticised law banning homosexuality.

The GA's main output are resolutions, essentially non-binding statements expressing the opinion of the assembly. Most pass or fail with little fanfare but occasionally resolutions spark angry political fights. Perhaps the most famous is a 1975 resolution which condemned Israel and declared "Zionism is a form of racism".

The GA serves as a permanent meeting place where all the world's nations are represented and truly global issues can be discussed. This year's 69th meeting of the GA is focused on climate change ahead of a Paris summit next year to thrash out the next big climate deal. "To ride this storm we need all hands on deck," Ban Ki-Moon, the UN secretary general, told leaders.

One of its greatest successes has been the setting of development goals, targets on reducing poverty, spreading education and empowering women that all of humanity is in theory committed to. While in reality, many countries have not lived up to their commitments, the goals have provided a framework to focus worldwide development efforts.

The interior of the United Nations General Assembly (ALAMY)

The GA does not make decisions about war and peace. Those responsibilities are given to the UN Security Council, the 15-member body whose resolutions can give legal authority for countries to go to war.

The Security Council is dominated by "the Big Five" - Britain, the US, France, China, and Russia - who have permanent seats on the council and can veto resolutions they disagree with. However, the GA is responsible for electing the ten other members of the council. This sometimes leads to battles where the West struggles with China and Russia to move their allies into the open seats.

Every year, the GA session follows a similar routine. Its occasionally worthy work on development goes largely unnoticed while its ugly rhetorical spats sometimes make headlines.

But there is a reason that world leaders labour over their annual speeches to the UN General Assembly. The global body may be a flawed institution but if you have a case to put before the world, it is still the best forum to make it.