Like all computers, tablets, and smart devices, Cisco devices require a CPU to execute OS instructions, such as system initialization, routing functions, and switching functions.
The CPU requires an OS to provide routing and switching functions. The Cisco Internetwork Operating System (IOS) is the system software used for most Cisco devices regardless of the size and type of the device. It is used for routers, LAN switches, small wireless access points, large routers with dozens of interfaces, and many other devices.
The highlighted component in the figure is the CPU of a Cisco 1941 router with the heatsink attached.