To play a video or replay a video can mean the exact same thing or different things. To play a video can mean for the first or hundredth time. Yet to replay the video definitely wouldn't be the first time.
Playback can mean the first time in some contexts. It is more general than replay. Replay would be hard to use for a first time viewing, but playback speaks more to the action of playing than the number of times a clips has been seen. The actual playing itself is altered by the speed control here. That is why the simpler 'play' was not used. Play is general, playback is more specific to the action of playing, and replay would be even more specific and pointed toward a subsequent viewing.
Mat