The question might seem naive, I know what a DNS server is, but as far as I know, in most cases, you don't need a DNS server as you (or the computer/server) can request an external DNS when needed. And yet, Ubuntu server includes Bind9 by default.
For instance, is there any advantage not getting rid of it in a typical LAMP stack? Is there any caching for reverse or something? Otherwise, I don't see any reason I shouldn't:
apt-get purge bind9 bind9-doc