Why do authors and TV show writers implement "foreshadowing"? Wouldn't you want to completely take your audience by surprise?

Hmm. I think this is up to the writer. I think this is more what the writers style is.

Foreshadowing can be used as a way to tease what is to come without revealing the full details or leaving out some key details.

An example that popped in my head is the teaser of the pilot to Big Little Lies. We see someone is dead at the bottom of the stairs. We get the sense it’s a party with lots of people standing around.

But that’s all we know. We don’t know who is dead - was he murdered? (Probably) Did he jump? If murdered, who killed him? What led up to the murder?

Then the season has eight episodes to explore those questions. It hooks the audience viewer in so they can find out these answers.

What would the pilot have been like without the foreshadowing teaser and it just started at the beginning. Would it have been as intriguing? Would it have hooked so many viewers in those first couple of minutes?

It’s really writers choice and the tone you want to set up. By starting with the foreshadowing, we understand it’s a bit of a mystery. If it didn’t start with the dead body, we would have no idea what we were building towards. Would it have packed as much of a punch? Maybe. Maybe not. Again, it depends on the writing.

(With this series, it’s also a whole different conversation of what they can do since it was based on a book. But say for the sake of this question that it wasn’t based on a book.)