A writer makes a promise with every word they choose. The opening line is no exception. It could be the reader's first interaction with that writer. It could also be the last.
That opening--that hook--should give the reader some insight into what kind of story they can expect. It should give hints about the tone and content of the piece. It should draw the reader in and give them the confidence to go forward. If it's really, really good, it might tell them the whole story.
The Magicians opens with, "Quentin did a magic trick. No one noticed." If you've read the book(s) or seen the show, you know how perfect this. Quentin is a chronically (possibly clinically) depressed dude for whom magic solves nothing. He spends much of the story doing magic and not feeling noticed.
Ride the Train by Shannon Iwanski starts like, "Carla stared at the non-descript white card she held in hand and read the words silently again, knowing that simple act was enough to condemn her to death". In that line, he presents is with the main conflict and gives us a lot of tension. We immediately know that, whatever is going on, it's life or death. And that danger drives the reader on.
"Someone was after me," Joan D. Vinge wrote in Catspaw. It's simple, but, like Ride the Train, it opens with some danger. And like The Magicians, it tells us what's going to happen in the rest of the book.
Shirley Jackson sets up the strange psychological world of The Haunting of Hill House with, "No love organism can continue for long to exist sanely under conditions of absolute reality; even larks and katydids are supposed, by some, to dream."
All of these lines are great. More than just grabbing attention, they make promises.
Jack's been working on his novella, Thirteen Hearts to Start a Storm. He had this hook, he was really proud of it. "The great Lake Pontchartrain Causeway had survived stronger hurricanes than this, but it cracked under the weight of the werewolves warring with an undulating mass of rats along its spans."
That is a damn good line, right? He had to cut it because, while technically true, all that stuff was happening in the world it's all in the background. The werewolves are not the focus of this book.
And that's where the promise comes in. If a writer promises me werewolves and does not deliver werewolves, readers are going to feel lied to. It's kind of a bait and switch.