DNS Propagation — Why Your New Site Is Not Live Yet
After you point a domain at a new host, it works for some people and not others, apparently at random. This queries the record against many resolvers at once and shows who has the new value. Nothing is being pushed anywhere — each resolver simply holds its cached answer until the TTL expires — so what you are watching is a set of independent caches expiring at different times.
When it helps
Because during a launch you need to know whether to wait or to fix something, and those look identical from your own machine. If most resolvers have the new value and a few do not, that is caching working as designed. If none of them do, the change never took effect — edited in the wrong zone, at a registrar that is no longer authoritative, or on a provider you migrated away from and forgot. Your own browser cannot distinguish those and will happily show you a cached answer for hours.
Worth running automatically
A launch is a window rather than a moment, and somebody has to keep checking until it closes. Scheduling it across that window replaces refreshing a page with a definite answer to the only question that matters — has everyone picked this up, can we turn off the old host. Turning it off too early is what converts a clean launch into a partial outage for whoever is still holding the old address.
What you get out of it
Tells you whether to wait or to go and fix something, which is the one thing you cannot determine from your own browser during a launch.