The belief here is that the keywords you use in H2-H6 subheadings are more heavily weighted in Google’s algorithm than words in plain text and that tags are therefore a ranking factor of their own.

In a Webmaster Central hangout, someone asked if it mattered if a page used an H2 heading tag instead of an H1. “A page without an H1 title will it still rank for keywords which is in the H2 title?” ...