Malcolm Turnbull is now in London for the biannual Commonwealth Heads of Government Meeting – known as CHOGM (and pronounced Chog-um).

The summit will run until April 20 and comes at a “crucial time”, according to the Prime Minister.

But what is this lesser-known summit all about? And, after all these years, is it still worth the effort?

Who will be there?

The 53 member nations of the Commonwealth represent about 2.4 billion people and are spread across six continents. Aside from Australia, it also includes the UK, New Zealand, Canada and South Africa.

Most of the nations are former British colonies, and a great bulk of them are in the Pacific and in Africa.

The 2018 summit’s credentials were boosted by Narendra Modi’s decision to attend, making him the first Indian PM to attend since 2010.

The Queen will also be there. It is expected to be Her Majesty’s last CHOGM – and there will be debate about whether Prince Charles should replace her as Head of the Commonwealth or the role should be democratically elected.

Soon-to-be-married Prince Harry and Meghan Markle will lead CHOGM’s youth events while Prince William and Prince Charles will take part in the summit.

What’s on the agenda?

The theme of this year’s CHOGM is “Towards a Common Future”.

Post Brexit, the UK has been talking up the Commonwealth as a new trade frontier.

Currently, only two of the UK’s largest 20 trading partners are Commonwealth nations. But the UK is hoping to cut free trade deals with Canada, New Zealand, Australia and India to make up for its departure from the EU.

The bad news for the British is that Mr Turnbull is more likely to focus on trade after CHOGM when he meets German Chancellor Angela Merkel to advocate for a free trade agreement with the EU.

Instead, the PM emphasised CHOGM as an opportunity to discuss recent events in Syria and other security challenges.

In London, Foreign Minister Julie Bishop got on the front foot on Wednesday by pressing the UK to spend more of its aid budget in the Pacific.

Australia has reduced its development budget in recent years, which has created a vacuum that China has sought to fill.

Pacific island nations will again press their richer Commonwealth cousins to lift their game on climate change.

Global warming threatens the existence of member states such as Kiribati and Vanuatu, where the summit was originally planned until the country was hit by Cyclone Pam.

A report released on the eve of the summit by Christian Aid suggested Australia – alongside the UK and Canada – were in climate mitigation “deficit”, while poorer Commonwealth nations were doing more of the heavy lifting on a per capita basis.

Mr Turnbull made no reference to climate change in his statement before leaving for London.

CHOGM … what is it good for?

Supporters of the summit argue it is an opportunity for like-minded nations – particularly the UK, Australia, Canada, New Zealand – to turn their minds to global challenges of trade and security.

For a ‘middle power’ like Australia, it is a chance to expand its influence, particularly into areas with emerging economies, such as Africa, they say.

But others have suggested the summit is a relic of a bygone era – even labelling it “the zombie summit that will not die“.

Paul Keating seemed to question CHOGM’s importance while PM, once telling other nations the summit was too long and held too frequently.

Asked by a reporter how his comments had been received, Mr Keating replied: “I am not sure all that well, but the fact is it had to be said and I said it.”

The key problem, critics say, is that aside from shared history there’s no longer much in common between the diverse member states.

For example, while Australia joined the UK, Canada and New Zealand in legalising marriage equality last year, same-sex relations are still illegal in 36 Commonwealth countries.

“We demand a lot from our leaders, but to foist CHOGM onto them really counts as a special type of punishment,” wrote former foreign correspondent Daniel Flitton in the Lowy Institute’s The Interpreter blog this week.