Summary is in fact important. The summary you enter for a Hub goes directly to a description meta tag of a web-page on which the hub is actually to be found. To see that - you can do the following: write a summary to any of your hubs, then open the published hub which has a summary. Once it is opened, if you use Chrome - do the right click and chose something like "show source code". After that you will see your hub from the inside. Press Ctrl+F - to open the search console, and type "description". You will see that the summary you've entered is right there, inside the description tag.