They need to know what the storms are doing at ground level. Ground-truth spotter reports allow meteorologists to correlate what they see on radar with what is actually happening. The reverse is true, as well. Consequently, the meteorologist is learning in real-time. These are known as severe weather events.
Flash floods are another classic severe weather event.
The greater the lead time of the warnings, the greater the chances that people can seek shelter. Obviously these reports will help meteorologists determine the effectiveness of their watch warning service. These numbers are incorporated into NWS messages that are disseminated on computer wire services to the media, businesses, government agencies, and the general public. In order for weather spotters to pass onto the NWS accurate and timely severe weather reports they need information on what to look for, and how to differentiate between the real and false severe storms, funnel clouds, tornadoes, etc. As with anything else, if one wants to be an effective spotter, they must attend several spotter classes.