It is considered good practice to provide a summary for every edit, especially when reverting (undoing) the actions of other editors or deleting existing text; otherwise, people may question your motives for the edit. Edits that do not have an edit summary are more likely to be reverted, because it may not be obvious what the purpose of the edit was.
Accurate summaries help other contributors decide whether it is worthwhile for them to review an edit, and to understand the change should they choose to review it. When a major edit (e.g. deletion of a substantial amount of text, a significant addition, or a substantial rewrite) doesn't have an edit summary, there are fewer reasons to assume good faith and busy editors may be more inclined to revert the change without checking it in detail. Summaries are less important for minor changes (which means generally unchallengeable changes such as spelling or grammar corrections), but a brief note like "fixed spelling" is helpful even then.
To avoid accidentally leaving edit summaries blank, you can select "Prompt me when entering a blank edit summary" on the Editing tab of your user preferences, if you have created an account.