Basically, all SEO writing techniques provide a cumulative effect. The search engines figure that hyperlinked keyphrases are important, and they'll give that phrase more weight. In fact, now that I'm on my soapbox, I think that every hyperlink on your home page should include a keyphrase. Both your headlines and your subheadlines should include at least one keyphrase. The reason to do this is the same as the reason for hyperlinked keyphrases.
Search engines think that headlines are important, highly relevant text.
The more keyphrases you have in your headlines, the more chances you'll have that the search engines will sit up and take notice. Yes, with modifications. This may work for the search engines, but it's horrid for your prospects. Search engines spider the entire text. For maximum effectiveness, I would recommend spreading your keyphrases throughout your body copy.