Genuinely comprehensive content naturally includes related terms as a byproduct of actually covering a topic thoroughly, which makes semantic breadth a useful proxy for depth.
Depth Produces Related Vocabulary Naturally
Writing that genuinely explores a topic's nuances tends to organically include the related terms a search engine would expect to see.
Shallow Content Often Lacks This Breadth
A thin page built around a single keyword, without real exploration of the topic, typically shows a narrower range of related vocabulary too.
Use Related Terms as a Self-Check for Coverage
If your draft is missing several obviously related concepts, that's often a sign the content itself needs more depth, not just more keywords.
Next step: Use the LSI Tool to identify related concepts your content might be missing before you consider it finished.