DNS or Domain Name System is one of the base system service in TCP/IP. It is used by email system, windows active directory, any web access and so on. Without DNS, a user will need to know the IP addresses of all the services s/he uses.
Understanding of DNS is a must in managing network system. As mentioned, almost all network service require the service of a DNS. A trouble happened in DNS will make everything not working properly. Using a DNS tools like nslookup may be useful in troubleshooting this problems.