A Hub Summary is not only important, it is mandatory. If you don't have a Hub Summary you will notice a violation cue for that Hub in the "My Account" (Stats) section which can be accessed at the top right of any Hub page. If you don't create one by yourself, the summary that shows up is automatically generated and may not be what you would want it to look like.
A Hub Summary is used to tell the people that may come across your article in search engines what your article is about. Think of it as a preview to your Hub like a movie trailer for a movie.
For more information about Hub Summaries and how to set them up, follow this link to an article from the HubPages Learning Center: http://hubpages.com/community/Learning-Center-How-...
I hope this answers your question!