Why can Javascript do for me?

So, I’ve learned HTML and CSS, and I know a little Javascript. I was wondering when I really will need to know javascript. i mean, parallax and smooth scroll are two cool effects, and so are js animations. But when would i need to do as much with javascript as with html or css on a professional website? Thanks in advance.
BTW, any project ideas would be appreciated.

Wow, thanks. I’ve got my work cut out for me.