She wasn't built up to anything, she was a missing person that the characters where looking for. Once they found out she was dead, she stopped being an important part of the story and just became a motive to stop Jefferson.
I always thought stopping the storm was a little more important though, considering that's how the entire series started, and it felt like Rachel fit in on that level. She didn't need to be important in relation to Jefferson. The murders almost seemed like a symptom of what was also causing the storm, and it was important to find out what was tying it all together and fix it.