A seed router is a L3 device you can use to discover networks and subnets. It's generally easier to use a seed router if you don't want to manually enter in all of your networks and subnets for the scan.
You can use the scan to discover any and all devices that are online. Generally, it's better to specify that the devices respond to WMI and SNMP as well, otherwise you'll get every single thing that responds to ICMP added to your list and you have to dig through it all before you can add them into NPM and start polling them.