The heading levels (1-6) are often considered a way of describing and determining the “importance” of a heading, with h1 being the most important. This might be reflected in the visual appearance of headings—higher-level headings typically appear as bigger and bolder text than lower-level headings.
But you can't just put an “important” heading anywhere. Its heading level relates to how much content it refers to, and that has to do with structure.
Reflecting structure accurately allows screen-reader users to use their screen reader's heading-navigation feature to navigate by heading level, building up an idea of document structure. It also ensures that, for people who use custom stylesheets, heading structure is preserved even when the appearance of headings is changed.... Read more about ✎ Technique: Heading structure
For a readable interface, it's important to use visual cues to group or separate content to show which content relates to other content. This makes content easier to read, especially for people who have difficulty reading on-screen content and people who use screen readers.
When grouping content visually, you must make sure that you're not skewing the source order and breaking up the same content for screen-reader users.