Value for followers
You should do it a few times whenever you're logged in. It gives value to those who are following you -- but only when you are retweeting good stuff, of course. So you should be choosy about what you retweet. Don't do it merely for the sake of it.
And giving value to your followers is important. By repeatedly sharing quality content you are reminding them that you're a good source of information and they'll keep an eye out for you. Then when you come to tweet, say, one of your own blog posts, they will be that much more likely to click on the link.
Simple and quick
Retweeting is also very easy to do. While you can cut and paste the tweet itself and put the RT in front of it, as many people prefer to do, you can also just press a button. That's quicker. And you are still getting on the radar of the person you retweeted.
Being retweeted is a buzz
Those whom you've retweeted will definitely appreciate the fact that you've done so. So they'll be more likely follow you back if they haven't already. They'll be more likely to retweet you after a while, too. And if some of those who do this are big time influencers with massive followings, it can help lift your profile immensely on the site.
Leads to more engagement
Quite often they'll say thanks for the RT. When they do, that's a really good sign. It proves conclusively that they've noticed you. Whenever they do this -- particularly the first time -- it's a good idea to say "my pleasure" or words to that effect. Since you've conversed with them already, it'll be much easier to strike up a conversation with them in the future, too.
Clearly, retweeting confers benefits all on its own. It's also an excellent complement to other forms of engagement. (To learn more about using Twitter check out the excellent ebook Twitter Dummy, listed in the right panel.)