Via: the proxies a message passed through

Each intermediary appends one entry naming the protocol version it received on and either its host name or a pseudonym. Reading the list from left to right walks the path in order, so a response with three entries went through three hops, and the last one is the closest to the origin server.

The loop detection use is the reason the header is required rather than optional for a proxy. If a proxy finds its own name already in the list, the message has come back to it, which means a routing misconfiguration somewhere ahead. Without that check a misconfigured pair of proxies can bounce a request between them indefinitely.

When debugging a cache it answers the first question, which is whether an intermediary was involved at all. A response arriving with a Via entry you did not expect usually means a corporate proxy or a transparent cache on the path, and that is often the explanation for content nobody can reproduce from outside that network.

Pseudonyms exist so that recording the path does not require publishing your internal topology. A gateway can call itself something meaningful to your operations team and opaque to everyone else, which keeps the loop detection working without listing real host names on the public internet.

It also has a request side that is easy to forget. The header travels in both directions, so a request arriving at your origin with entries already in it went through intermediaries before it reached you, and that is worth logging next to the forwarded address headers. Together they answer whether a strange request came through your own edge or around it.