We know why they say this, because it sounds very c ol :) to write a compiler, but the reality is that it is a transpiler.
For more please read:
It is important because Dynamic Programming Language do not need to be compiled, they have interpreter. Although we have compiler optimization phase within V8, but still there is no need for a user to use a tool for compiling. Execution happens at runtime.
The short story is that before ES6, major browsers were ahead of standard, so all of our code could be run.
But after ES6 not all browsers completely support ES6!
A tool to convert ES6 code to ES5 that all browsers can parse. That is it.
Update: Mon Sep 23 2019 14:10:08 GMT+0330 (Iran Standard Time)