Ask HN: Explain to me why functional programming is better like I'm stupid

I have a coworker who really harps on functional programming. To me the block of .map(x).ifelseget().etc() is so much harder to read than defining and mutating variables, if/switch statements, and loops. Also, if you get a stack trace, the line number doesn't really tell you much if you are doing 4 things on a single line.

I have always wondered this since I have been confronted with it, but in an interview recently I was given a light docking for not using functional programming. Why is it better?