This page in a nutshell:
When editing a page, a small "Summary" field under the main edit-box looks like this:

It's good practice to fill in the Edit Summary field, or add to it in the case of section editing, as it helps everyone to understand what is changed, such as when perusing the history of the page. So the recommendation is, always fill in the summary field. This is considered an important guideline. Even a short summary is better than no summary.