To avoid having an image you're actively using being removed from the registry. Arguably it doesn't happen often, but when you're running something in production you should be in control. Below a certain scale it might not make sense to run your own registry and you just run the risk, but if you can affort it, you should "vendor" everything.

Not Docker, but I worked on a project that used certain Python libraries, where the author would yank the older versions of the library everything they felt like rewriting everything, this happened multiple times. After that happened the second time we just started running our own Python package registry. That way we where in control of upgrades.