In the beginning of the video, the narration talks about the spillway plug being a feature designed to erode in a fashion that delayed/slowed the inevitable failure of the entire dam. In this case, the spillway plug helped the dam release water in a semi-controlled fashion over a series of hours, instead of an uncontrolled fashion in a matter of minutes.

If a embankment dam is overtopped, it is going to fail. They designed it with a slightly lower section on one end so they could be sure if it failed, it failed in a predictable way.