Data link layer deals with more hardware part of the network. It performs tasks such as Collision detection and collision avoidance which is out of scope of network layer. Network layer is the most important layer which keeps internet intact but it cannot do the tasks which link layer can do and vice versa. Now the question comes to your mind that why these 2 layers can't be merged? These two layers are almost working together for example, while fragmentation network layer will not know the MTU if there is no DLL, DLL is responsible from where Network layer gets the MTU and do the fragmentation, and the final point of yours that router is responsible for routing the packets on the internet yep that's true, but its not routers job to get the data from your device. It is layer 2 devices such as a switch, its job of a switch to connect a device to a network. When you are buying a router you are not just buying a router but also it contains a switch which is responsible for data to transfer from your Ethernet port to router's Ethernet port.