It separates the coder from the engineer. It asks you to stop worrying about whether a function returns a string or an int , and start worrying about whether a billion people can wish their mother a "Happy Birthday" without crashing the planet.
Then, the interviewer says: "Let's move to the next round. How would you design YouTube?" what is system design interview
You've got this.
Embrace the ambiguity. Draw the boxes. Talk about the trade-offs. And remember: The interviewer is not your enemy; they are your pair programmer. Let them help you. It separates the coder from the engineer