go - Why is time.sleep required to run certain goroutines?

Question

In the GO tutorial, we have this slide: Goroutines

package main

import (
    "fmt"
    "time"
)

func say(s string) {
    for i := 0; i < 5; i++ {
        time.Sleep(100 * time.Millisecond)
        fmt.Println(s)
    }
}

func main() {
    go say("world")
    say("hello")
}

Running this code produces expected results ("world" and "hello" written to the screen interchangeably 5 times).

However, if we comment out time.Sleep (and consequently, the "time" line of the import) and run the program again, we are left with only "hello" written to the screen five times.

What is so important about time.Sleep that saves the goroutine from dying?

Answer 1

Waitting for answers