Not necessary

If you need use a new approaches with JS, arrow functions, classes etc... you can use ES6. I am using in my personal projects. At work, we use to backend service with NodeJS.

If you are using ReactJS, I think (IMO) it is necessary. Why would you not use it?

ES6 is the most flexible language I have come across in my short career so far. The syntactic sugar that this new version of JS has is well... how you say it.. mind = blown

if you don't use ES6 what is your other option ?

for learning yes it is necessary to learn the new things about your field, but if you have project built with using ES5 or less, necessary or not it depends on your project.

Necessary - no. All current browsers can run pre-ES6 JavaScript without transpilation; and it is possible to write whatever you need without ES6.

I'd recommend it though, even though that means you need to transpile your production code so it works in all browsers (eg. with Babel).

This is particularly true if you are learning JavaScript now - if that's what you mean by "start"? You might as well learn ES6 as the majority of browsers support it now and it is the ongoing standard. So you may as well learn how to write JS in ES6 style and not the older style.

not necessary but its easy to adopt. Also it will reduce your code structure