Earthquakes are a great example. If I feel an earthquake, I might visit a site like that to get the scientific facts about the epicenter and magnitude of the earthquake.

However, there is a totally different kind of value to Twitter posts: instead of data, I'm getting firsthand accounts of who felt it, where, what they were doing at the time, and what effect it had on their life. Yes, this information is (obviously) anecdotal, but its also interesting, informative and immediate.