The relationship between Namecoin and Bitcoin are of a symbiotic nature. Namecoin is merged mined with Bitcoin so hashing power that secures the Bitcoin network is used to secure Namecoin. The benefit of having a separate blockchain is that all of the DNS entries and other data for Namecoin do not bloat or otherwise affect the Bitcoin blockchain. Changes can be made to the Namecoin network without interfering with Bitcoin.
Namecoin puts a monetary incentive for people to participate in the network which supports a DNS protocol backed by a distributed system. This means third parties can't simply seize domains anymore.