Sending data involves many separate jobs: encoding bits, addressing machines, choosing routes, reaching the right app. The OSI model splits these into 7 layers so each can be designed and debugged on its own. Use it when you need to place a protocol or device, like saying a router works at layer 3. It is a teaching and troubleshooting model; the internet itself runs on TCP/IP, so do not claim real stacks have separate session and presentation layers.