secondary vs. delegation

[Brian Noecker]

I've gotten myself confused a bit on justifying delegation.  We do DNS
service for a company foo.com that wants to run their own DNS servers for
internal corp.foo.com.  We are looking at delgating the corp.foo.com to
their name server so they can administer the zone file how they want.  Their
internal corp.foo.com servers are all internal IP address 192.168.x.x
servers.

My question is, why justification is there to delegate rather than to just
be a secondary for a subdomain?

Thanks in advance.