Percussive therapy devices do something. What they do is smaller and shorter-lived than the marketing implies, and the mechanism is almost certainly not the one on the box.
What the evidence supports
Short-term range of motion. The most consistent finding. A couple of minutes on a muscle increases how far the joint moves, immediately, without the strength cost that static stretching carries. That makes it genuinely useful in a warm-up.
Reduced perceived soreness. People report feeling less sore. Whether that is analgesia, distraction or expectation is hard to separate, and for practical purposes it may not matter.
Both effects fade quickly. Measured hours later, most of the difference is gone.
What it does not do
Break up lactic acid. There is nothing to break up. Lactate is cleared within an hour or two of finishing and has nothing to do with the soreness you feel two days later, which is muscle damage and inflammation. See how long DOMS lasts.
Break up scar tissue or fascia. The forces required to deform fascia mechanically are far beyond what a handheld device produces. Whatever is changing, it is your nervous system’s tolerance rather than the tissue itself.
Speed up tissue repair. No evidence supports this, and the mechanism would have to be circulatory in a way the timescales do not fit.
How to use one, if you have one
Before training, on the muscles you want range of motion from. This is where the evidence is strongest, and it is a better warm-up tool than a recovery tool.
One to two minutes per area. That is the protocol length in the research. More is not better.
Not hard enough to hurt. Pain is not the mechanism, and bruising is a real possibility on lean areas.
Not on joints, bone, the neck or anywhere numb or injured. The obvious safety envelope, frequently ignored.
Where it sits in the hierarchy
Below sleep, below eating enough, below managing training load, and roughly level with foam rolling, which it resembles more than manufacturers would like. If your recovery is genuinely poor, a device is not the lever. The signals that tell you which lever it is are in the signs of overtraining.
The related question with a similar answer: does stretching prevent injury.