Divide and Conquer. They provide abstraction layers that you can individually reason about, individually implement, independently exchange... In the real world,

Divide and Conquer. They provide abstraction layers that you can individually reason about, individually implement, independently exchange... In the real world, no abstraction is perfect: There will always be "leaks" between the abstractions, optimizations that partially dissolve them etc., but it's a good principle and allows vastly more complexity than without. The problem with the ISO/OSI model is less how to fit real network protocols into it, it's more that it actually was proposed as a model for real network implementations, but TCP/IP, which does not fit well into the model, grew and displaced most other protocols and became the de facto standard for networking. This happened for multiple complex reasons that are not all technical.