It depends on the stack used for a product. A frontend can be rendered in the browser using React/Angular/Vue.js/etc or server-side using frameworks like Wicket/Velocity etc.

Depending on the frameworks used it can be quite useful to know a little about back-end tech. I've done projects where the front-end developers had no back-end experience which made in depth discussions very difficult.

IMO a developer should at least have minimal knowledge of tech used in a project no matter what their work area is. This makes discussions easier and gives everyone the ability to chip in when needed.