A network map shows the devices that are physically connected to the network, and includes some basic information about each device. This can help in troubleshooting networking problems. The Link-Layer Topology Discovery Mapper service probes the network looking for these devices. It does not require IP, and the quality of the network map increases as the number of devices that ‘respond’ to the service increases. Here’s a PowerPoint presentation explaining it further. This service is generally only useful if you are experiencing network trouble. Though it can safely be disabled, since it only runs when needed (manual startup by default) there is little reason to disable it.