The . zone describes all the root name servers available worldwide. This zone is required if your DNS server needs to resolve queries outside of the authoritative zones for which its responsible -or- if you are simply running a caching-only name server. i.e. no authoritative zones other than localhost, etc...
A good "visual" example of a DNS query using the the root name servers (aka recursion) is to type:
dig +trace
www.example.com
Note the query starts at "." (the root) then kinda works in reverse to ".com" then "example.com" and finally "www.example.com" i.e. recursion
