Adding your network devices to ISE is optional as you can create a default device profile however it is required for various ISE features and I always ensure my devices are added. Adding the devices allows you to specify the type of device and where the device is located which is information you may use within ISE policies for TrustSec / AAA.
Logging into the ISE admin GUI you need to navigate to 'Administrator -> Network Resources -> Network Devices', once loaded click on 'Add' to be presented with the below page. You are required to give the device a name which I always keep the same as the device hostname, the device IP address which is also the IP address ISE expects TACACS / RADIUS traffic to be sourced from. The device profile which for me is Cisco and also the device Type and Location. The device type and location are custom fields and you can create new Device Types and Locations by clicking on the orange arrow. Within my network I have all my sites added under Location and for Device Types I currently have 'IOS' and 'WLC' although you may have more.
I have also ticked the TACACS box and defined a shared secret that ISE should check for when receiving TACACS requests from this device.