Some areas within the forecast area have little or no sources of weather information. The spotter reports help fill those gaps that radar and satellites can't cover. The spotters' data, which includes location, elevation and a description of the severity of the conditions, can help the NWS formulate and pinpoint advisories or warnings. When appropriate, the NWS gives the information to flood control agencies, the state Office of Emergency Services, the county Office of Disaster Preparedness, various police and fire departments and other agencies that deal with weather-related emergencies. Sometimes, the NWS will call a spotter for an observance if radar or satellite images indicate something could be happening in an area, even if there have been no reports.
Spotters can and do provide valuable confirmation.
The NWS' Skywarn system is a similar but more sophisticated and proactive spotter network. During severe weather outbreaks, the NWS activates Skywarn, and spotters in target areas are notified. Skywarn spotters are then asked to relay observances from locations near potential trouble spots.