<form id="foo"></form>
<div class="target">...</div>
<div class="doesnt-matter">
<input type="checkbox" form="foo" required>
</div>
#foo:invalid ~ .target
1. The input element(s) can now be anywhere on the page. You can, for example, use adjacency to style a control button, without torturing your HTML to squeeze the button into being a sibling of whatever it's activating.
2. You can use form:valid as a parent selector i.e. as a wrapper element. You couldn't use an input:checked as a parent selector because the <input> element's content model is "nothing" i.e. there are no child elements under an <input>, but you sure can with a <form>. The result is less brittle to code-rot.
This is extremely freeing and allows you to use, for example, a :checked adjacency for styling an activation button and a wrapper element (a form:valid) for the large complex component (modal? drop-down? slideover? video content?) that it activates. Heck, it can even be an iframe. Here's the bones of an example:
https://codepen.io/inopinatus/pen/vYxrOeR?editors=1100
There are two downsides:
1. Behaviour of some elements is different inside a <form>. Most obviously, don't nest a <form> inside with the same scope.
2. Your HTML maybe become a mess of scattered <form>, <input>, and <label> elements that many other developers will look at and say "WTF was this person thinking" and "what does this button do?"