Put simply, you want to be seen! You want popularity and you want your audience to grow. Face it, 99.999% of tweets aren't being read by anyone and aren't even being shared outside of your own audience. Why did you even join twitter in the first place? You did it as a means to market yourself!
You wouldn't be on this page if this wasn't the case. Try this, go to Twitter.com and search for @your_twitter_name. Do you see any results coming up? Most likely you'll only see your avatar popping up and occasionally some retweets in between.
Now imagine this!
You search for your twitter name and you see thousands of people sharing your tweets to their audience. People
usually only retweet the good stuff, so their audience is more likely to click on the link or read the tweet than any other tweets. This means that your audience will grow exponentially and your credibility as well.