Lucy is the first life-changing event in Saint Maybe for Danny, Ian, and the Bedloe family. Her introduction to the family is what changes their destinies for years to come. Not only does her potential infidelity lead to Danny driving his car into a wall and dying, but her eventual death leaves Ian responsible for their three children.
Everything might have worked out differently if Ian hadn't suspected that Daphne wasn't actually premature. When he sees her, he immediately thinks that she's too large and well-developed to be a seven-months-old baby. He realizes that Lucy and Danny didn't even know each other nine months ago and immediately suspects that Daphne might be someone else's child.
One night, while he's babysitting, he repeats these concerns to Danny. He tells him that he doesn't know if Daphne is his child. He says that he's suspicious of Lucy, who is supposedly out drinking with girlfriends. Danny doesn't take this information well. He can't live with the possible infidelity of the woman he loves so much and the realization that his child might not actually be his. He ends up driving his car into a wall and dying.
Without Lucy, none of these events would have taken place. Danny would have likely been in a different relationship that didn't lead Ian to have suspicions about the woman's fidelity. Everything would have worked out completely differently.