A lot of webpage developers use the H1/H2/H3 tags for formatting. Nothing wrong with this but if you talk about SEO, these tags are very important. These tags support the overall theme placed in the title tag.
Imagine for a moment that your webpage is an outline and the topic of your outline is the “title” of your page (the one in your title tag) and the H1 tags are your first level topics under your title. Now H2 tags are sub topics that support whatever is in H1.
To give a example, here’s a sample web page outline with the proper SEO use of the H1/H2 tags.
<title> Nokia N95</title>
<h1>Nokia N95 Features</h1>
<p> some intro paragraph here</p>
<h2>Nokia N95 multimedia features</h2>
<p> talk about camera, video, display features</p>
<h2>Physical Features of Nokia N95</h2>
<p> talk about physical dimensions, color, style..etc</p>
<h1>Nokia N95 Accessories</h1>
<p> talk about n95 specific accessories</p>
Notice that the overall subject and keyword being targeted is “Nokia N95″ (which is the keyword in the title). H1 tags such as “Nokia N95 features” and “Nokia N95 accessories” all support the main subject. Then, supporting the H1 tags are the H2 tags.
Make sure however that the supporting content which is the meat of the webpage is well written, and contains important information about your topic. Always keep in mind what sort of information people who are searching for “nokia n95″ are expecting to find. If within 30seconds, they can’t find what they’re looking for in your webpage, they would definitely leave your site immediately.