When you're creating a Web page, you are using HTML which is a structured markup language. This structure makes it possible for Web browsers to view your content, cellphones to view the content, and screen readers to read the content aloud. Plus, search engine robots use the structure to add your Web pages to their indexes.
While it's certainly true that a page can rank high in search engines and have no structural tags in them at all. But chances are that this is because those pages are already very popular and have a lot of backlinks to them that get them a better rating. As a Web designer you can choose to ignore structural tags altogether and focus only on backlinks. But why? H1, H2, and H3 headlines are easy to add to your pages, and with CSS they don't have any detriments. And since they help search engines categorize your pages more efficiently, you'll rank higher in search engines and thus get backlinks more quickly.
H1 is the Most Important Phrase or Sentence on Your Page
If you think of a Web page as an outline, the H1, H2, etc. heading tags serve to divide the page into sections. Your most important headline is your H1 headline. This generally indicates the topic for the entire Web page and is where most people look first when they're trying to figure out what the page is about. Since search engines first priority is to provide search results that people want, they try to use the same techniques to determine what a page is about. So content in the H1 tag will be considered most important and given a slightly higher rank than other content on the page.
If you're trying to optimize a Web page for a specific keyword phrase, this is where that phrase should appear, preferably the first two words or so. This tells the search engine that these words are important - that this is what this page is about.
H1 Heading Tags Provide Structure and Pages Without Structure are Harder for the Robots to Read
When you structure your pages logically with one H1 tag, one or two H2 tags, and one or two H2 tags beneath the H2 tags, you create a page that is structured and easy to follow. Search engine robots are a little more simplistic than most people, but they are trying to index pages with the keywords that the pages are about. And pages that are structured to make it obvious what they are about (and the keyword phrases associated with them) will rank better than those that are not.
Images for Headlines is Just a Bad Idea
Many people like to use images instead of H1 tags for their headlines. This is not accessble so search engine robots can't read the text in the image. So you get a double-negative. The robots can't read the headline and there's no stucture. If you must use images for your headlines, consider using some form of image replacement so that you can still use H1 tags.
No One is Forcing You to Use H1, H2 or H3 Tags
Ultimately, it's up to you whether you use H1 tags to format your pages or not. If you'd rather spend your SEO time doing something else, then that's fine. But if you're going to spend a lot of effort creating DIV-heavy pages just to stick CSS styles on your headlines, then you might as well use H1 - H6 headlines instead. You'll have a page that has fewer characters and is structured even if the customer can't use CSS.
