I assume this question is based on the logic that an
<h1> tag represents the one main title of the page. While that may have been true in previous days, HTML5 has changed that type of logic. In HTML5, the
<h1> tag is the main title of a section. This could be a full page, a subpage, a module, etc. A good example of why this difference matters is when building a modular system of different sections dedicated to different activities.
Imagine a system with multiple "pages" lined up next to each other. Each page serves a different purpose: one for the weather, one for organizing tasks, one for display customers, etc. Each of these pages can be further drilled down into (hour-by-hour forecast, single task, single customer) and these "subpages" could be visually placed alongside the actual pages.
Before HTML5, you would need to logically determine which header tag was appropriate and, if you went more than 6 levels deep, you ran out of header tags. With HTML5, you can set them all as
<h1> without extra logic or a nesting limitation.