The root font size is applied to the HTML tag. That is where all text in your site is based on. If you increase the root font size, all of the em, rem and % based text will increase in size relative to the root font size.
To understand this best, you need to grasp the concept of modern responsive typography.
Here's another one.
The Body font-weight applies to regular text in your site like the text in your text widget, regular blog post not created with Cornerstone, the Classic Text element and all of the text that will inherit the body font settings.
The Heading font-weight applies to regular headings like the post title, widget title and all the text that will inherit the following selectors
h1, .h1, h2, .h2, h3, .h3, h4, .h4, h5, .h5, h6, .h6.
If you're wondering why you need to setup V2 font size every time, this is because there is a plan for a template manager. Once this feature is realized, you will be able to create presets so you just have to select a preset and not set individually each time. Please read this news and I encourage you to join the discussion here.