You don't "need" an application framework.

People have been building websites (even SPAs) before the 2010s and they didn't "need" these frameworks.

Given what we know today, I'll grant you that in some cases people would want to use a framework, but it's hardly a necessity unless the context provides more specific requirements.

you don't "need" a framework for backend development, too, but most people find it highly convenient

Its can be convenient to have some kind of reusable conponents. Like listviews wtih certain functionality. Same for routing. Caching etc.

Native web components / custom elements work great for this.

So you end up with an component framework.

You dont ”need” anything. But most people prefer some organization, familiarity and reusability, and not having to reinvent the wheel in every project they start.