if you know Microsoft DFS, namespace consupt is very similar to it. (but it only inside a cluster for one vserver)
You have a volume called volA you can mount them under any path eg:- /departmentA
NAS Clients will never know /departmentA is called volA (they see only departmentA)
Otfourse this is not the only reason namespace exists.
For example, if you want to create a directory structure like the above picture
You can create those volume and mount each volume using the appropriate juncation-path.
And you can share just the top level directory as one share.
The advantage here is you have the flexibilty of netapp flexvol for each one of them.
And it could be in different aggr and different nodes. there for the load on the share will
be more managable for the storge admin.
There is lot more to namespace.. to understand everything I recommend you attend
a NCDA training or read a lot of documentation.