The Network layer is used to route physical data to the destination point, it decides the actual path from where data travels to destination point. The network layer supports both Connection oriented and connectionless services.
Today, the modern computers are multitasking and at a time different applications try to send and receive data so the Transport layer is given the charge to convert all the data that is received from the application into packets and these packets have headers which contains the information such as where to send the data etc. But Transport layer provides the physical path of sending data so; it is difficult to combine these two layers into one.