Owl: A subway train covers 2.4 km in exactly 3 minutes between two stations. Average speed: 0.8 km/min. Must the speedometer have read exactly 0.8 km/min at some moment in between?
If the train's position function is continuous on the closed interval and differentiable on the open interval, the Mean Value Theorem says yes — some c where s'(c) equals the secant slope.
MVT guarantees existence; it does not always hand you c on a silver platter. And without continuity (or differentiability), the promise can fail.
Sketch the secant from start to stop, then imagine a tangent parallel to that secant. Where does that picture live in the hypotheses?