TweetLater has a number of useful features. You can schedule tweets (twitter posts) to occur in the future. For example, I like to find useful quotes on BrainyQuote, then queue them up and post them once a day on Twitter. Interestingly, these tend to get retweeted (reposted) fairly often, and thus get wider circulation on Twitter than just to my followers. Others may follow me because of them.
You can also tell TweetLater to autofollow any new followers or to hold them for your approval first. I prefer to use their vetting process to see who the person is before following them back as they might be spammy, incoherent, or writing in a different language.
They have lots more features too, especially in the pro version.