For example you can do a drop down menu in css. Then you have some new requirements that css alone can't do it. Even a basic thing, could be impossible in css.

For example you can do a drop down menu in css. Then you have some new requirements that css alone can't do it. Even a basic thing, could be impossible in css. All your css logic goes to trash. Or you can keep css logic and make a mess of a software If you have done it in js, you'd be able to reuse some code.