Absolutely it can improve traffic! I think you need a balance of two things within your summary to have a great one.
1) Use your main keyword phrase once for SEO purposes. Closely related keyword variations can work good if incorporated in a way that works well with point two.
2) Let the reader know what your hub is about and why they should come read it without giving them the answer they are seeking.
Quite often your summary is what shows on the search engines so it's a great chance to catch the person's attention and get them to click.
I say a balance because if it's just full of keywords the search engine may love it and list it higher and all but if the reader doesn't click what does it matter? If your summary is only targeted at the user but doesn't include your keywords and thus doesn't rank high enough to be seen and clicked then once again it doesn't matter. Balance is key here in my opinion.
Some people will say that the summary isn't meaningful for SEO purposes, but I don't believe that at all. Maybe I'm wrong here?
Everything I say here applies even more so to the Title, except you must keep your Title much shorter than the summary. No one disputes the SEO value of the Title and many SEO experts still say it's the most important factor to ranking highly when it comes to on page optimization. Summary would be top ten.