Katie receives a phone call that scares her, this starts a series of spooky events. This scene is important to the plot because of the suspense it causes. It also creates a tension between Bill and Katie that lasts throughout the story.
Bill's death gives Katie a reason to be upset and causes more tension within her. She hated feeling alone especially with the events before that day. This also lead her to forget all about the phone call.
Katie sad and broken in Bill's old study calls her past self to try and warn her to take Bill to the hospital. This creates a weird almost spooky feeling run through Katie. Without this scene we would have never known that the person sobbing on the phone was Katie herself from 5 years in the future.