I cant think of One Main Reason. there are many few i can list.
Unless you wish to change the interpretation of your assigned network number, you do not have to define a subnet mask. The decision to subnet is commonly driven by topological or organizational considerations.
The topological reasons for subnetting include:
Overcoming distance limitations
Some network hardware has very strict distance limitations. Ethernet is the most common example. The maximum length of a "thick" Ethernet cable is 500 meters; the maximum length of a "thin" cable is 300 meters; the total length of an Ethernet, called the maximum diameter, is 2500 meters. If you need to cover a greater distance, you can use IP routers to link a series of Ethernet cables. Individual cable still must not exceed the maximum allowable length, but using this approach, every cable is a separate Ethernet. Therefore the total length of the IP network can exceed the maximum length of an Ethernet.
Interconnecting dissimilar physical networks
IP routers can be used to link together networks that have different and incompatible underlying network technologies.
Filtering traffic between networks
Local traffic stays on the local subnet. Only traffic intended for other networks is forwarded through the gateway.
Subnetting is not the only way to solve topology problems. Networks are implemented in hardware and can be altered by changing or adding hardware, but subnetting is an effective way to overcome these problems at the TCP/IP software level.
Of course, there are non-technical reasons for creating subnets. Subnets often serve organizational purposes such as:
Simplifying network administration
Subnets can be used to delegate address management, troubleshooting, and other network administration responsibilities to smaller organizations within the overall organization. This is an effective tool for managing a large network with a limited staff. It places the responsibility for managing the subnet on the people who benefit from its use.
Recognizing organizational structure
The structure of an organization (or simply office politics) may require independent network management for some divisions. Creating independently managed subnets for these divisions is preferable to having them go directly to an ISP to get their own independent network numbers.
Isolating traffic by organization
Certain organizations may prefer to have their local traffic isolated to a network that is primarily accessible only to members of that organization. This is particularly appropriate when security is involved. For example, the payroll department might not want their network packets on the engineering network, where some clever person could figure out how to intercept them.
Isolating potential problems
If a certain segment is less reliable than the remainder of the net, you may want to make that segment a subnet. For example, if the research group puts experimental systems on the network from time to time, or experiments with the network itself, this part of the network will be unstable. You would make it a subnet to prevent experimental hardware or software from interfering with the rest of the network.
Also you may find more information on the below link.
http://articles.techrepublic.com.com/5100-10878_11-5034563.html
Regards,
Pravin..
