What is the H1 tag?
Technically, it defines the largest header out of the group of HTML headers that range from <h1> to <h6>.
Html headings should be used for good content structure, ergo anything that is good for content structure is very good for Search Engine Optimization. Concordantly, you should always apply them to any page that has content and use your main keywords within them, Vis a Vis the search engine spider’s focus on these terms.
It’s up in the air weather the H1 tag has any more power than the H2 or H3 etc., but technically you should always use H1 as your main header and subsequently use the less numbers for sub headings.
As well as being great for search engine optimization, the correct application of Heading tag also is a great usability tool and allows readers to scan your content without having to read every single paragraph to help them find the content that they are interested in.