As far as I know a dns server states on which server a domain is hosted
I was wondering what's the point of geographically dispersed dns servers?
I mean, if the domain is on a server in california, once the dns have fully propagated the websites are reachable, why should a 3rd, 4th etc dns exist (apart in case the others are down) in australia or europe?
The main point of multiple DNS in dispersed locations is to avoid a single point of failure (though that's not probably what you are looking for)... If one DNS server goes down, there is another one which is still available to respond where to find your site. The more you have, the less chance of a DNS failure you have.... beyond that, I don't really see the benefit
and having servers in 2 different datacenters would benefit from it?
say server1.domain.com is in DataCenter A and server2.domain.com is in DataCenter B, would this setup benefit from having 2 dns in area A pushing visits to DCA and 2 dns in area B pushing to DCB?
or it wouldn't make sense at all?
The point of having multiple servers in different locations is to prevent the case when perhaps a disaster occurs in a Datacenter - or even in a city. Both DNS servers should be reporting the same information for a domain. So ideally you would want a DNS server for your domain in DCA and DCB to prevent a singly point of failure. DCA goes out, DCB can report your domain information or vice versa.
Having different a DNS server closer to the target area could increase the reach speed, however it is unlikely to create a large difference. Additionally, you would have to set up the DNS servers accordingly knowing the target area for a site (i.e. knowing you are targetting australia, so using australia dns entries)... not exactly the easiest thing in the world, but still doable.
Each DNS entry you have will have a TTL specified (Time To Live) which will specify how long the entry is good for,. That is, when you lookup a site for the first time your ISPs DNS servers will query your site's DNS server. It will then cache the information for the TTL time (or at least it should, it's not required to)... so beyond the first resolution, your DNS server usually doesn't even get contacted because your ISP will have the DNS for the site Cached!