Delegating DNS

Why would you delegate a DNS zone? If a DNS server is being overwhelmed by traffic, it would make sense to delegate a portion of its namespace to another server; if a DNS server is separated from important "clients" (e.g. Exchange servers, or many workstations) over a slow WAN link (as a way of moving the most important (popular) DNS server closer to its base); or if you just need to shift some of the administrative work to somebody else. Hopefully I'll be able to grasp the the concepts being tested on in 70-297, but in real life, I think I still feel very fuzzy on why/how the whole DNS delegation thing exists/works.

When I created a new zone on my test DNS server, I found that unqualified hostnames failed in nslookup. Using group policy (Computer\Admin\Network\DNS) I added an entry to the DNS suffix search order for the zone that had previously failed the nslookup. After fixing a subnet mask on my test workstation (oops) and rebooting (to apply the machine-level group policy), it worked!