Its a fairly important metric. The more incoming links you have the more you know your url is out on the net somewhere trying its best to drive people to your page. Backlinks are one of the factors in SEO that search engines take into consideration. It also shows how well fans are making links/sharing your hub on the web to others.
The higher the number of stars the better. Hubs can be very successful with just one star and just having traffic from search engines.