Why we use JavaScript.

Answer 1

Mainly to make webpages more flexible (aka more dynamic instead of static) by adding tools/functions into it

Answer 2

To make your website more advanced (CSS and HTML can't do that)

Answer 3

Javascript makes the page dynamic and allows reaction to the user input.
for example you could write a javascript function that triggers as soon as a user enters a string in a text box, that would modify the page or auto complete other fields depending on the input.