Take a private Class A address 10.0.0.0/8 for example, too many host and not enough network address.
You have many rooms with up to around 200 computers and you are only given that 10.0.0.0 address to use and it would be much wiser to use because it keeps the routing table small and if you ever heard of route aggregation or supernetting you would understand why. http://techrepublic.com.com/5100-1035_11-5825449-2.html#
Anyways you have about 200host in many rooms so subnetting a Class A address you would borrow 16bits from the host portion so now one of the network would appear as 10.0.0.0/24 which would give you about 254 address for your host and the second network would appear 10.0.1.0/24 which would give you 254 address in another room and so on.
Subnetting gives you more control over the network address and help segregate the host when using a router or not.