It stinks to be afraid, but fear also has an important use: reminding us to get the heck out of dangerous situations. For example, if Martians are firing their Heat-Ray at you, maybe you shouldn't just stand there (as the narrator does). Maybe you should drop to the ground and crawl away. But just because fear can be useful doesn't mean it will be. After all, there are times when you might be too scared and do something really stupid. For example, if Martians fire their Heat-Ray at you, but you're in a crowd, don't all try to get away through a narrow area, because someone will end up trampled. Yeah, we don't always think clearly when we're afraid. That's the negative side of fear. And watch out, because it can be contagious.
Wells uses The War of the Worlds to show that fear is an emotion that can only be successfully managed by individuals who have special training. By contrast, groups always react to fear in the worst way possible.
In The War of the Worlds, fear is closely related to the idea of folly, in that fear can help us overcome our folly.