Port scanning is appropriate when you plan to use the NMS for service-level monitoring. Limiting discovery by IP address range or subnet can avoid searching networks that you don't own or control. The others used discovery to populate maps, typically as a hierarchy of connected subnets. A root map provides an eagle-eye view, while subordinate maps are used to drill down and isolate problems. Finally, few networks are static.
Devices come and go and get swapped out.
New services are added, sometimes without authorization. Knowing what your network contains on any day can be a challenge. Products that support incremental, scheduled discovery make maps much easier to maintain. What We Tested Pt. Network Discovery Pt.