Have you read it? It's just a really interesting story that holds up to this day, first of all. So that would account for some of its popularity. It also gave examples of moral ideals for the Greeks, as well as examples of poor behaviour (Paris). Also had religious/mythological significance.

It's a story that has been used by a wide range of people for a wide range of purposes. For Vergil, the story served as part of a foundation legend that tied Rome into existing bodies of Greek legends. For many western mediaeval writers, the story served as a way of confirming their own nations' positions as inheritors of the Roman legacy. For Chaucer and Shakespeare, it served as a way of recreating ancient genres and solidifying their own works' reputability. For Wolfgang Petersen, it served as a comment on the relationship between personal identity and reputation, and an excuse for lots of CGI triremes. For the poet of the Odyssey, it was a mechanism for talking about a prehistoric prototype for contemporary colonists. For the poet of the Iliad it was something else again, for the poet of the Aethiopis it was something else again, for the poet of the Cypria it was something else again, and so on, and so on.

Everyone who had a hand in the transmission of the legend, whether you like the resulting treatments or not, had their own perspective on the purpose or meaning or moral of the story. That applies to the Iliad just as much as it does to Petersen's Troy.

None of these is a definitive picture of these artists' intentions of course: in each case, it's just something characteristic that happened to leap to my mind. There doesn't have to be a single correct way of interpreting the purpose of a work of art. Every time anyone reads the Iliad, it's an interaction between the reader and the poet, so the "meaning" that emerges is always something different. (There are of course correct and less-correct ways of interpreting its significance as a historical document... but that's a separate matter.)

Does every novel have to have a single specific moral? Does every movie have to have one specific allegorical meaning? Of course not. Why should the Iliad?

Having said that, of course there are things we can say about some unique characteristics it has as a work of art of its time. On one level, the conflict between Achilles and Agamemnon documents a class conflict between those who inherit power (Agamemnon) and those who are productive (Achilles). On another level it documents the friction between different ethnic groups in the Troad, in a period when Greeks were colonizing an area that had a pre-existing non-Greek population. On yet another level, it is programmatic for a pan-Hellenic perspective on what it means to be "Greek".

And again, none of these is exhaustive: there are plenty of other ways of interpreting the literary impact of the Iliad. Literary works can't be pinned down to a single pithy moral. And if they can, they're probably rather shallow.

There is a consensus that behind the Illiad and the Odyssey lies some truths. But before entering the subject itself you have to be conscious that the Illiad does not tell the trojan war. It only focus on a few days of the last year while the greek are besieging Troy for more than 9 years. Those two stories are those wich survived, other stories about other heroes existed.

The second important elements is that we can trace the story of the Illiad to the IX century BC while the war presumably happened somewhere around 1200 BC. Indeed the Acheans are described as wearing bronze armors, while in the IXth century Greece had entered in the Iron age. So from the start it's a poetic stance on past events, made to enable the (aristocratic) listeners to identify with the heroic figures.

The third element is the poetic nature of those texts. Both are in verses, and part of the text that we know show that before being written down in the VIth century it was sung. This particularity was identify in the 1920 when Milman Parry studied the oral tradition of epic poetry in Bosnia and made a parrallel with the Homeric question. He identified repetitive elements always made of half a verse. Those elements like Achilleus swift-footed, helped the singer to remember the story and to make better verses. This mode of transmission via professionnal singing poets also explain why there are repetition of verses in the text known in antiquity among other anomalies like the two travels in the underworld of Ulysses to see his father and ask him about the next step in his journey to come home while the second happen after the reunification with his wife and son. The museion (Museum, the institution that contained the library) of Alexandria played a big role in those corrections.

To conclude the Trojan War did happened but it could have been quite different than a 10 year siege. But in it are traces of a time when instead of fighting together in a phalanx, fights would be conducted by champions.