In a nutshell: people's entries answer the question, "What are you doing?"
I like it. It's NOT text-messaging; if that's what you're using it for, that would be incredibly inefficient. It's micro-blogging: keeping a blog (personal journal) where the entries are quite short. You can do it from anywhere- computer, phone, iPod, anything with an internet connection.
It's yet another way for people to interact with each other without that socially awkward act of face-to-face talking.
*I say as I post on an internet message board full of complete strangers*