The obvious use: a visual effect
The most familiar use of reversed video is the creative effect — undoing an action for comedic timing, an object reassembling itself, a splash flowing back into a glass. This trick works especially well with actions that have a satisfying mirror when undone: dropping something, spilling liquid, jumping, or any motion with a clear beginning and end state.
It's also a genuinely useful loop technique: playing a clip forward and then reversed back-to-back creates a seamless loop with no visible jump at the repeat point, since the last frame of the reversed section matches the first frame of the forward section perfectly.
A less obvious use: fixing footage you can't reshoot
Sometimes the only usable take of an action was filmed in reverse by necessity — a subject moving from B to A when you needed A to B, or camera positioning that only worked filming the action backward. Reversing the footage after the fact fixes the direction without needing to reshoot, provided the action itself looks acceptable played backward, which isn't true for every kind of motion (walking backward looks noticeably different from walking forward, for instance).
This is a legitimate editing technique in professional filmmaking too, not just a novelty trick — reversed and then flipped footage is a known workaround for certain shots that are easier to capture in one direction than the other.
What happens to audio when you reverse a video
Reversing the video track also reverses any audio that came with it by default, and reversed speech or music is almost always unintelligible and often unintentionally comedic rather than usable. For most reversed-video projects, muting the reversed section or replacing the audio entirely with something intended to play backward (or simply new music) produces a far better result than leaving the original audio reversed.
Not every clip reverses convincingly
Motion that looks natural forward can look distinctly unnatural backward — anything involving gravity (falling, pouring, walking) tends to look obviously wrong in reverse because the physics reads incorrectly to the eye, even if viewers can't immediately articulate why. Motion without a strong directional or gravitational component — abstract movement, some types of dance, camera pans across a static scene — tends to reverse much more convincingly.
It's worth previewing the reversed result before committing to using it in a final project, since what works as a reversal is genuinely dependent on the specific content of the clip rather than being universally predictable.