Networking modules are hardware devices that can be used to add interfaces to existing network systems. This is usually done by installing them into a switch or router. A network module could be an ethernet module or a Fibre Channel (FC) card for example, which acts as an adapter that connects your device to a network via the new interface.
When you buy a router or switch, it comes with default interfaces and some empty slots. Networking modules can be put inside the empty slots to expand the functionality of the router or switch. Unlike networking modules, i/o modules act as an interface between the central processing unit (CPU) and memory and also between one or more peripherals.
Networking modules allow you to expand the performance and bandwidth of your network appliance according to your needs, for example, if you wish to connect fibre cables to your router, you can use a networking module with fibre interfaces, or if you wish to establish a Wide Area Network (WAN) connection between two different locations, you can use a WAN networking module.