What was the reason for this side story in the campaign?
Maybe to provide some more information as to what happened before the UNSC was on the ground when the Covenant was invading the city. And maybe to make a sympathetic story about an 18 year-old girl and what she had to go through when the Covenant started invading, watching Brutes murder civilians, seeing what happens when people start to think the world is ending and starts looting, being hunted by Kinsler, seeing a mentally-insane Marshall gun down several NMPD Officers, the SWAT team killing the Marshall, and witnessing Corrupt cops fire into a crowd of defenseless civilians.
Bungie wanted to give players an idea about what was going on in the city before the Covenant had shown up. It also shows that Humanity had discovered the location of the Artifact (portal to the Ark) just before the Covenant invasion.
I have to agree with both of these ideas.
I personally felt that it gave some impact to the play space itself.
I also, liked how it gave you subtle hints as to why certain bodies were strewn about.
Thanks guys for the replies it has enlightened me