DNS has two kinds of servers with very different jobs. Authoritative servers are the source of truth for the zones they host: asked about example.com, they answer from the zone data with the "authoritative answer" flag set, and they do not look anything up elsewhere. Recursive resolvers do the searching: they walk from the root servers to the TLD servers to the domain's authoritative servers and cache what they learn.
A zone is normally served by several authoritative servers that hold identical copies, traditionally one primary where edits are made and secondaries that replicate from it. Large providers serve the same zone from many locations using anycast.
Querying an authoritative server directly shows what the zone contains right now, without caches in between. Comparing that with what public resolvers return is the quickest way to tell whether a problem is in the zone itself or is just an old cached answer.