Different betweet using sqlbrite and content observer

Question

I'm reading the intruduction about sqlbrite and RxJava. I'm wondering what's the difference between using sqlbrite and content observer to do something like montoring sms database's changes. What I've known is, maybe, in sqlbrite, Observables won't send until at least one Subscriber subscribes them, but in normal content observer, observerables always send messages. Correct me if I'm wrong and are there any othere differences? Thanks a lot.