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Schmidhuber's formulation of curiosity and interestingness as a (possibly the) human learning algorithm. Now when someone says "that's interesting" I gain information about the situation, where previously I interpreted it purely as an expression of an emotion. I still see it primarily about emotion, but now understand the whys of the emotional response: it's what (part of) our learning algorithm feels like from the inside.
There are some interesting signaling implications as well.
I would be wary of concluding too much from phatic statements. "That's interesting" is more likely to be a phatic utterance than not in some contexts/with some people