NOTE: The following is a theory based on facts gleaned from reliable sources.
Master Chief and the Forward unto Dawn were not left behind as the portal collapsed. They might have been redirected (as many fans believe) to new coordinates set by Mendicant Bias, the Forerunner AI. These coordinates turned out to be close to the Forerunner planet of Requiem, as we all know.
The possibility of portal redirection during it’s activation can be seen in the second halo: CEA terminal as well as in Halo Legends, where the halo array, in formation above the ark, is sent through the portal all at once. This is proof that the portal can be redirected to at least 7 coordinated while it is activated, meaning that there is a possibility that the aft section of the forward unto dawn was sent to another location in the galaxy. This would disprove that Requiem is outside the galaxy and close to the ark installations.
Also, being sent through the portal to a distance at least 25,000 light years (effective range of the installation 04 and it’s replication on the ark, 04b), is the only way that the master chief could have survived it’s firing.
As to the planet’s purpose, having recently reread the Forerunner saga books by Greg Bear, I believe that Requiem is the Forerunner Domain, or the source of all Forerunner knowledge. This would fit with why the Didact would be present there in his new cryptum (the glowing and flying sphere seen on the E3 stage demo), as the Domain was inaccessable to the Forerunners at the time of the activation of the Halo array. The Didact may have referred to his mission to reactivate and and preserve the Domain in his last recorded words “and we will take the Great Journey without you.” Some possibilities are that the Domain cut off connection to the Forerunners during the Flood conflict to protect the secrets of it’s location and prevent the flood from accessing it via infected Forerunner hosts (if the flood accessed the Domain, they would know EVERYTHING about the Forerunners). It is possible that the Didact made his mission to preserve the knowledge and legacy of his people after the firing of the Halo Array, and also brought all the remaining Forerunners (biologicals like the life workers and warrior-servants on the ark, and the mechanical Promethians AI’s seen in the E3 stage demo) to achieve his goal.
To add to this, the namesake for the planet, a “Requiem,” is a mass to the dead. This may be some more evidence that the Didact and the remaining Forerunners retreated to Requiem after the firing of the Array to preserve their culture and to make sure that those lost in their war with the flood and the firing of the array wouldn’t have died in vain.
That is all.