The answer to that, according to band member Harry Styles, is a chance to peek behind the curtain and see what really makes One Direction tick.
"With social media and 10-minute interviews you do with people there is only so much you can get across in terms of your personality," he said ahead of the film's London premiere this week. "We just want to show what we are about, and get across what we are like and what we are like with each other. I think that is the kind of stuff the fans want to see - the behind-the-scenes stuff."
One Direction: This Is Us does indeed offer insights into the crazy life of the biggest boy band on the planet, from their exhaustive touring schedule to more intimate moments with each other and family members.
In three years, the British five-piece - Styles, Niall Horan, Zayn Malik, Liam Payne and Louis Tomlinson - have gone from early X Factor rejects to Simon Cowell-backed global super-stardom, with hits including What Makes You Beautiful and Live While We're Young, and whose every move is scrutinised by the media and coveted by their passionate fan-base.
But what emerges is a portrait of a group of young boys, still aged between 19 and 21, who live to have fun and who have managed to keep their feet on the ground, no matter how crazy things become around them (one scene shows them getting mobbed while out shopping in Amsterdam).
For director Morgan Spurlock, a filmmaker better known for his political activism and anti-fast food documentary Super Size Me, this was part of the appeal of getting involved with them in the first place.
"One reason why I think they have been so incredibly successful with the fans is that they are so incredibly grounded and that is what comes off in the movie," he said. "There is no air of superiority, there is no air of success that permeates this movie. You see five guys who are the same five guys they were three years ago. I think that normality is what has continued to propel them to where they are now."
The biggest challenge Spurlock faced was fitting everything in. "There are five stories to tell in basically 62 minutes - 28 minutes of the film is concert and 62 minutes is behind-the-scenes documentary footage. So, we are telling five stories over about 12 minutes each and there is quite a bit to try and squeeze in.
"For us, it was about really telling their stories, what they were going through, the struggles they face, how they look at and deal with fame, and the relationships they still have with each other and their families."
Critics of the film may argue it still plays things very safe, refusing to really dwell on the hardships faced by such punishing tour schedules or the temptations that almost inevitably come with musical success. The obsessive nature of fans who recently became the subject of controversial Channel 4 documentary Crazy About One Direction is also largely ignored.
But when asked about the lack of a rock and roll lifestyle, or the behaviour of some fans, the boys do get agitated.
Styles, especially, dismisses the Channel 4 documentary by insisting that "it's better just to ignore s**t like that", while Liam added: "I think that was a load of tripe. To be honest, we all think the fans are amazing and the amount of dedication they show is second to none. They are the best fans in the world."
Spurlock concurs, saying of the fans: "The thing I thought was incredible was that you suddenly realise this is not just a British or an American phenomenon. Everywhere they go, they are being chased by hordes of fans. We were in Mexico City and there were 5000 people camping outside the hotel. This is such a massive global phenomenon and it is only growing."
As for the suggestion that their lifestyle lacks a certain sex and drugs factor, Louis quips: "We like to think we're rock 'n' roll but we're not really."
This is certainly borne out by footage showing the band performing various good deeds, whether it is dressing up in disguise to meet and greet fans as they enter a concert venue, or heading to Africa to support Comic Relief with the charity single One Way Or Another, or even in Zayn's case buying a new house for his mother.
Was seeing his mum moved to tears at all difficult for him to watch? "It was strange to film it because obviously that was the first time I had spoken to my mum about it and it was the first time she had got into the house," he recalled. "But it was good and it is an amazing thing for me because I can look back on that in 15 years and watch this amazing footage."
Indeed, as much as the film serves as a behind-the-scenes treat for fans, it is also an enjoyable reminder for the boys themselves of some of the things they have experienced to this point in their careers, as Louis noted: "Because so much is happening to us and we kind of just lose it all, it's great to be able to sit back and watch it and remember and relive those moments."
Does this mean another One Direction film could be heading our way some time soon?
"I think we just want to see how this one goes," answers Liam. "It's so nerve-wracking making a film. I was thinking the other day that when we sit in the premiere and everybody is watching it on the big screen it's going to be amazing. But maybe if people like it - I'm sure we've got time."
One Direction: This Is Us opens in cinemas on August 29