An exact-match keyword is the precise phrase you're targeting; LSI keywords are the surrounding vocabulary that gives that phrase context and depth.
Exact-Match Keywords Anchor the Page
Your primary target phrase still belongs in key places like the title, headline, and opening paragraph to clearly establish what the page is about.
LSI Keywords Build Out the Substance
Related terms and subtopics fill in the depth around that anchor, showing genuine, comprehensive coverage rather than a single repeated phrase.
Neither Works Well Without the Other
A page with only exact-match repetition reads as thin and forced; a page with only related terms and no clear anchor can feel unfocused.
Next step: Use the LSI Tool to build out the related-term vocabulary that supports your exact-match target phrase.