I bet Twitter didn't think that preparing possible MBA students for the B-school applications should be a part of the services they provide, but it's a positive externality of their service. Having just those 160 characters to write your tweet prepares you for the questions where you have 185 characters to describe your activity or organization and your impact while in the group. This would be a rather daunting task without the practice provided by Twitter.
I often start tweeting without really thinking of how to say what I need to. Often I exceed that 160 character limit on my first try and have to actually think about what's most important in my potential tweet and how to say it more tersely. It's that skill of editing on the fly that has turned out to be most helpful for these very short paragraphs on my B school apps.
Who ever thought that Twitter would provide it's users with real world skills? I guess anything's possible. Keep those tweets coming to improve your marketable skills!