When you already have a SAN, you would introduce a NAS device when you have some specific file sharing requirements.
The NAS device provides significant advantages in simplicity and administration for file sharing. The SAN will already handle the high performance database accesses that you may have so the NAS would fit very well in environments such as engineering, development, etc. where file sharing is very important. NAS really started in those environments and have been very successful there. The simplicity of administration has been very valuable in environments where there are not any storage professionals.
This was first published in August 2001