I’m going to guess that the most common usage of the term is in the phrase “I know,” as in:

1. “You have to be careful what you take Lipitor with.” “I know.”
2. “The science articles have gotten really hard to read in Wikipedia.” “I know.”
3. “This cookbook thinks you’ll just happen to have strudel dough just hanging around.” “I know.”
4. “The books are arranged by the author’s last name within any one topic area.” “I know.”
5. “They’re closing the Red Line on weekends.” “I know!”

In each of these, the speaker is not claiming to have an inner state of belief that is justifiable and true. The speaker is using “I know” to shape the conversation and the social relationship with the initial speaker.

1., 4. “You can stop explaining now.”
2., 3. “I agree with you. We’re on the same side.”
5. “I agree that it’s outrageous!”

And I won’t even mention words like “surely” and “certainly” that are almost always used to indicate that you’re going to present no evidence for the claim that follows.