You don't need a Page Builder to create a website. That said, with a Page Builder you can quickly and relatively easily create a beautiful collection page or landing page for your website. You can separately format or animate the elements (blocks) in your Page Builder. There are different types of blocks, for example for images or other media, text or icons.
Gutenberg, the WordPress editor, is in fact also a Page Builder: you can use it to build your static pages from different blocks. If you want some more options, you can install another Page Builder.
Free and paid
Page Builders are available for both systems, free and paid. What you can do with it has little to do with which CMS you use. The functionality differs per Page Builder, not per CMS.
There are simple and comprehensive Page Builders for both WordPress and Joomla. Some developers focus on one CMS (SP Page Builder for Joomla, Elementor for WordPress), others create Page Builders for both systems (Yootheme Pro for example).
Most Page Builders have a free (free / lite) version and a paid (pro) version. The difference is usually in the amount of elements (blocks, addons): with a free Page Builder there are about ten, fifteen, with paid Page Builders often more than fifty. In addition, support for free Page Builders is often limited, as with all other extensions, plugins and templates.
Is a Page Builder really necessary?
By default, Joomla has many options for whether or not to display elements on your site. You can set per module where it should be displayed, and there are generally many positions available within your template to place a module. If you know a little bit about how to build a page with modules, you don't really need a Page Builder for Joomla. With WordPress, the default options are very limited, so you will want to resort to a Page Builder faster.
Page Builders and performance
So a Page Builder makes it a lot easier to build pages, but some Page Builders can slow down your website significantly by loading all kinds of components and scripts that you may not be using at all. So before installing, try to find out what the Page Builder does for your performance.