This is necessary since if you have:

<div> --> ShadowRoot
  <div>Hello</div>
  <span>World</span>
<div>

and inside ShadowRoot

<content select="div"></content>
<shadow></shadow>

you cannot tell which nodes ended up in <shadow> without getDistributedNodes() or filling the pool yourself and then walking all insertion points and looking for the non-distributed nodes to notice the UA shadow with the <content select="*"> is being used and should select the remaining nodes (div).