What is the Chilcot inquiry into the Iraq war for?

Usually, politicians like independent inquiries - they use them to outsource and delay difficult decisions.

The risk, of course, is that they lose control.

And Sir John Chilcot has used every bit of his independence to defy the body politic and take his time over a report that many wanted published earlier.

Commons Speaker John Bercow spoke for many at Westminster earlier, when he told MPs: "Sir John should be aware that there is a very real sense of anger and frustration across the whole House at what seems a substantial disservice that has been done."

But that raises questions about what this inquiry is for.

If it's to provide closure for the families of the fallen, then it has so far failed in that task.

If it is to learn lessons, well, they are likely to be learned too late to inform any decision about military action in, say, Syria.

But if the inquiry is designed to find out what happened and hold people to account, then there is an argument for taking the time to get it right.

After all, the Saville inquiry into Bloody Sunday, a single incident, took 12 years compared to Chilcot's seven year study into the causes and consequences of an entire war.

Senior Tory MP Bernard Jenkin has some sympathy for Sir John.

"He has been heroic in the way he's resisted public pressure," he said.

"He's been resisting pressure from some of the witnesses.

"I thought the criticism levelled by Clare Short earlier this year was absolutely out of order, I think it's out of order for the former prime minister, Tony Blair, to be issuing his, sort of, "prebuttal" denials in advance of the publication of the report."

Yet public inquiries don't have to take this long.

The inquiry into the Hillsborough tragedy took less than three years, the Franks inquiry into the Falklands conflict took just six months.

And William Shakespeare managed to encompass the whole of life in a corpus of fewer words than the two million Sir John has ground out.

Yet those words will matter.

Many people have made up their minds about the Iraq war but others still want to know the reasons for what happened.

British servicemen and women spent six years fighting in those deserts. Sir John Chilcot is taking a little longer to explain why.