Explanation
A save is a statement of intent to use something later, which is a much higher bar than a like and a much better predictor of whether a video was actually valuable. Formats built to be referenced, explainers, lists and how-tos, earn saves at rates that passive entertainment content never reaches.
The practical use of the metric is that it tells you which of your content is useful as opposed to merely watchable. Those are different products and an account usually needs to know which one it is making.
The common mistake
Asking for saves on content nobody would use again. The request only works when the video contains something worth returning to, and asking anyway trains the audience to ignore your calls to action.