Twitter takes the idea that "if the service is free, you're not the customer, you're the product" to the extreme. Where you might complain about reddit or facebook that they're time-wasters or that the content quality is not as good as you might like, their purpose for end users is clear enough on its own. There are Twitter users who seem to enjoy it without it occurring to them to constantly question its purpose though: they're the users who enjoy feeling plugged into... other users who want to feel plugged into users who feel plugged into... It's sort of the online informational equivalent of bland pop music and summer blockbusters that no one takes in for the artistic merit but that you can reliably make conversation about.
I'll bet there's a spectrum from reddit to facebook to twitter users that maps pretty directly to how much extraversion is a part of someone's personality. Reddit is there for those of us who want to hyper-rationally compartmentalize, analyze and dissect everything at length, while those of us that are more concenred about cultivating a presentable real-life persona can take in quick sound bites and easily stay plugged into whatever the mass media machine and popular sentiment thinks is hot on Twitter.