SNMP or Simple Network Management Protocol version 2 is used by the Network Detective Data Collector to query the MIB 2 branch of network devices for system profile information. These network devices include, routers, switches printers or any other snmp enabled device on the network.
It is also used by the Inspector appliance to query for Layer 2/3 information in order to correctly map devices on the network. Running a simple network scan will not return layer 2/3 info. You must have an Inspector appliance to query for the snmp data needed to build the network maps.