DNS Security allows you to secure the DNS information, and trust that the IP address that the DNS resolver returned for an FQDN has not been poisoned. This is not needed for small deployments, but when you start crossing DNS administrative boundaries, it becomes higher priority.

Per wikipedia:

[DNSSEC] is a set of extensions to DNS which provide to DNS clients (resolvers) origin authentication of DNS data, authenticated denial of existence, and data integrity, but not availability or confidentiality.