It calculates the "in between" frames so the animation looks smoother.
(magma blocks and still lava use it, as well as a few others that I've forgotten)
Forgot? No. This was written before that was introduced.
What I lack is the time (and more importantly interest) to do intensive testing on how those frames are generated and what the limitations, quirks, and so forth of the system are. I mean, besides taking up more processing power than is strictly necessary. That one is obvious. But the details are sadly not so I haven't done those tests or added any information about it to the guide.