The description of your document is optional. You might not need it, because your title says it all, or your document is really short. In all other cases, a description should help to provide context for the readers, without having to read the whole document to see if it is relevant for them. The description is also searcheable, so if you want to add more significant keywords that don’t fit into your title, write them into the description. That way, your document can be found easily by everyone.
