Heading tags are used by search engines and screen reader softwares to construct an outline of the page.
Starting at <h1> and not skipping any level eases this automatic construction.
Noncompliant code example
<h2>My Title</h2> <!-- Non-Compliant - h1 is not being used -->
<h3>My Sub Title</h3> <!-- Compliant -->
Compliant solution
<h1>My Title</h1> <!-- Compliant -->
<h2>My Sub Title</h2> <!-- Compliant -->
