Also, if you have none present, Google will make up it's own based on what's on the page - sometime not always the right context - if you include your own, you're more likely to have control of what's displayed.
How to Do SEO in 2013
12/27/2012 Whether you're new in the game or a seasoned campaigner, you would have noticed by now that Search Engine Optimisation is by no means a walk in the park. Algorithm changes get made a multitude of times annually; 516 changes were made by Google alone in 2010, and they're just the ones they tell us about. As a result, staying on top of the game is quite a task for anyone, individual or team.