It may be of critical importance in some games that, no matter how low the framerate is, the player never misses an event due to skipped frames.
There are also games that are not real time even in their animations, and so there may be no benefit to skipping frames rather than just letting it run at whatever framerate. Slowed tick rate mostly feels weird if one has certain expectations for the passage of time.
There is no rule that the angles of a triangle add to 180 degrees. It only holds true in Euclidean geometry, which this is not.