When should you use a trait as a type in rust?

I recently started learning rust and I learned traits are valid types in rust. I thought this was pretty neat, so I made a little example program. What exactly is the point of having a trait as a type if it has very similar and or the same functionality as a method and also comes with the costs of

1. Must be wrapped in Box making it always heap allocated instead of stack allocated
2. Has an expensive initialization cost

I'm new to rust and don't 100% understand traits yet but a explanation would be nice

struct Person;

trait Speaks {
    fn speak(&self) {
        println!("Yo yiggity yo")
    }
}

impl Speaks for Person {}

impl Person {
    fn say(&self) {
        println!("Yo yiggity yo")
    }
}

fn main() {
    let john: Box<dyn Speaks> = Box::new(Person);
    john.speak();
    let carl: Person = Person;
    carl.say();
}

>>>Yo yiggity yo
>>>Yo yiggity yo