Her existence was to give the guy a reason to go into danger and to phone somebody about other things than "is she back!?", thus experiencing a little of what was happening on Earth chaos-wise.
A second possible existence reason may pertain to a sequel at some point? Fuck, I don't know... It's JJ Abrams. :P
- - - Updated - - -
That's the problem I have with this... is this even set on our Earth? This almost seems like a near future-earth of some kind...
The Original Cloverfield is clearly set on present-day Earth.
I really wish they didn't make more out of this franchise other than what the Original Cloverfield was meant to be - and that was a "Hey, here's a Kaiju movie told from an unknowing civilian's perspective!". >_<