It's good to have. This information comes up in the results of a search, either internally or via Google, Bing, etc. If you have it, then you're controlling the message. If you don't, you're leaving that descriptive information up to whatever logic the search engine is using. Most of the time, it's OK. But, sometimes, it turns out to be a jumbled mess that's not useful to anyone. While it won't play any role in ranking, we recommend having a relevant summary of your page in the meta-description.
If you would like to enter a description for a page, click on Settings while editing it, then on the following screen, enter a short description into the Page Description field that can be found there.