It is not completely necessary no, however it does offer some huge advantages in your build. I’ll try to explain.
For a long time Wordpress (WP) allowed you to create custom fields for your posts or pages. That feature was removed in favor of Gutenburg approach, where you add blocks and elements that are NOT equivalent to custom fields. The Advanced Custom Fields (ACF) plugin replaces that feature and it’s better than the old WP custom fields anyway.
The point of using custom fields is that you can create any layout you can imagine and display bits of content anywhere you need it. Otherwise you have to just output whatever is in the Gutenburg editor, referred to as the “content”. That’s fine for things like blog post, but not very flexible.
Since we can no longer add custom fields in WP without a 3rd party plugin, ACF is the best of them. You don’t need the pro version per se’. You can use the free ACF version. The ACF Pro version has some really nice features if you need those.
If you aren’t creating custom fields in WP, you’re probably working on a smaller site. Maybe using Customizer and just outputting the excerpts, content, and featured image fields. without the need for custom fields. Or maybe you are integrating a page builder like Elementor and you don’t need custom fields so much.
Hope this helps!