Sections are used to visually group fields in the editor and to control the section's fields when they are defined as repeatable. This is usable when you have a lot of fields and need to group them logically to avoid clutter.
If you need a section to appear (visually) as a "sub-section" of another section, prefix the section's title with a "-" (dash). This will indent the section in the editor, so it visually appears as a sub-section.
Sections without any fields will not be displayed.
Sections are not custom fields in their own right, so you cannot reference them via Liquid e.g. to loop over the contained fields