The layered concept of networking was developed to accommodate changes in technology. Each layer of a specific network model may be responsible for a different function of the network. Each layer will pass information up and down to the next subsequent layer as data is processed.
By separating the network communications into logical smaller pieces, the OSI model simplifies how network protocols are designed. The OSI model was designed to ensure different types of equipment (such as network adapters, hubs, and routers) would all be compatible even if built by different manufacturers. A product from one network equipment vendor that implements OSI Layer 2 functionality, for example, will be much more likely to interoperate with another vendor's OSI Layer 3 product because both vendors are following the same model.
The OSI model also makes network designs more extensible as new protocols and other network services are generally easier to add to a layered architecture than to a monolithic one.
The benefits to layering networking protocol specifications are many including:
Interoperability - Layering promotes greater interoperability between devices from different manufacturers and even between different generations of the same type of device from the same manufacturer.
Greater Compatibility - One of the greatest of all of the benefits of using a hierarchal or layered approach to networking and communications protocols is the greater compatibility between devices, systems and networks that this delivers.
Better Flexibility - Layering and the greater compatibility that it delivers goes a long way to improving the flexibility; particularly in terms of options and choices, that network engineers and administrators alike crave so much.
Flexibility and Peace of Mind - Peace of mind in knowing that if worst comes to worst and a key core network device; suddenly and without prior warning decides to give up the ghost, you can rest assured that a replacement or temporary stand-by can be readily put to work with the highest degree of confidence that it will do the job.
Even though it may not be up to doing the job at the same speed it will still do it; at least, until a better, more permanent solution can be implemented. This is a state of affairs that is much more acceptable than for a lengthy cessation of network services or assets unavailability to occur. 80% is oh so much more pleasing than 0%.
Increased Life Expectancy - Increased product working life expectancies as backwards compatibility is made considerably easier. Devices from different technology generations can co-exist thus the older units do not get discarded immediately newer technologies are adopted.
Scalability - Experience has shown that a layered or hierarchal approach to networking protocol design and implementation scales better than the horizontal approach.
Mobility - Greater mobility is more readily delivered whenever we adopt the layered and segmented strategies into our architectural design
Value Added Features - It is far easier to incorporate and implement value added features into products or services when the entire system has been built on the use of a layered philosophy.
Cost Effective Quality - The layered approach has proven time and time again to be the most economical way of developing and implementing any system(s) be they small, simple, large or complex makes no difference.
This ease of development and implementation translates to greater efficiency and effectiveness which in turn translates into greater economic rationalization and cheaper products while not compromising quality.
Modularity - I am sure that you have come across plug-ins and add-ons. These are common and classical examples of the benefits to be derived from the use of a hierarchal (layered) approach to design.
Innate Plasticity - Layering allows for innate plasticity to be built into devices at all levels and stages from the get-go, to implementation, on through optimization and upgrade cycles throughout a component's entire useful working lifecycle thereafter.
The Graduated, Blended Approach to Migration - Compatibility enables technologies to co-exist side-by-side which results in quicker uptake of newer technologies as the older asset investments can still continue to be productive. Thus migration to newer technologies and standards can be undertaken in stages or phases over a period of time. This is what is known as the graduated blended approach; which is the opposite of the sudden adoption approach.
Standardization and Certification - The layered approach to networking protocol specifications facilitates a more streamlined and simplified standardization and certification process; particularly from an "industry" point of view. This is due to the clearer and more distinct definition and demarcation of what functions occur at each layer when the layered approach is taken.
Task Segmentation - Breaking a large complex system into smaller more manageable subcomponents allows for easier development and implementation of new technologies; as well as facilitating human comprehension of what may be very diverse and complex systems.
Portability - Layered networking protocols are much easier to port from one system or architecture to another.
Compartmentalization of Functionality - The compartmentalization or layering of processes, procedures and communications functions gives developers the freedom to concentrate on a specific layer or specific functions within that layer's realm of responsibility without the need for great concern or modification of any other layer.