Every section that appears on a new page is required to have a section heading. This is for the purpose of identification and navigation in the navigation panel when previewing a smart online form.
If you do not want a section to have a section heading, you can hide the section heading. Note, you still have to provide a section heading that will be visible in the navigation panel, so some thought is still required for hidden section headings.
To hide a section, simply click the Edit button for the section (which looks like a pencil) and choose “Properties”. The edit panel will then open on the right where you can check "Hide Heading". On the left-hand side of the builder, the section heading text will become grey and italicised.
If a section is not on a new page, you do not have to give that section a heading. Giving the section a heading and then hiding is recommended for identification purposes when you or another person is building or editing a form.