Headings are the skeleton of a page. They tell a reader, and an engine, how the content is organised and what each part is about. Get the structure right and a page is easy to scan, easy to retrieve from, and easy to build a contents list from.
Use one h1, the page's main title. Use h2 for the main sections and h3 for the parts within them, in order, without skipping levels. A heading should describe the section under it, and where you can, phrase it as the question that section answers, because that is often the question a person will ask an engine.
Do not use headings for styling. If you want big text, use CSS. A heading that is not a real section confuses the structure, and structure is exactly what an engine reads to chunk your page.
This connects to answerability. A clear heading above a self contained passage is what lets an engine retrieve that passage as a unit. See how retrieval works. Topkay checks you have a single h1 and that headings run in order.