A cache doesn't move data. It copies it. And the moment a copy exists, someone has to decide how long it's allowed to be wrong.
Caching is one of the most reflexively-reached-for performance tools in networked systems — memory instead of disk, a CDN edge node instead of the origin server, a browser's local cache instead of a round trip over the network. In every case the mechanism is identical: put a copy of some data somewhere closer and cheaper to read than the authoritative source, so future requests are answered faster and the original source does less work. What gets skipped over just as reflexively is that this copy is now a second version of the truth, sitting apart from the source that can still change out from under it. Every caching layer, without exception, has to answer one question explicitly: what happens when the source changes and the cached copy doesn't know yet?