This page in a nutshell:
Always use edit summaries when you can.

When undoing an edit for any given reason, it's important to let the user that made the edit know why you are undoing their edit. For this purpose, there is the edit summary box located above the "Preview" and "Save page" buttons while editing a page. This can easily help prevent edit wars, misunderstandings and confusion, not only with the editors involved, but anyone who sees the edits. Do remember that not everyone thinks the same way or understands the situation as well.

This does not apply only to undoing other edits, as edit summaries can be equally useful for other edits. As an example, when removing a lot of content for being redundant or unnecessary, not everyone who views the edit will immediately realize what is happening and they may mistake it for vandalism. This is not necessarily the case for obviously constructive edits, such as fixing typos.

To make it easier for everyone involved, it is strongly recommended that you provide a summary for an edit, particularly when undoing another.