Although we often look at the more violent aspects of weather when we think of human fatalities due to weather (hurricanes and tornadoes make great new stories), the vast majority of human fatalities due top weather are from heat. On average heat waves and drought kill 100,000 people every year. Although some storm events can kill a great many people (the Bhola cyclone in 1970 killed 500,000 people in India) the yearly average is closer to 10,000 people.
The cost of damages due to tropical storms and tornadoes is a high number although probably not as high as the cost of lost crops due to drought, which maybe as high as 8 billion dollars a year. In fact the U.N. estimates that droughts are the costliest of all natural disasters (take that geologists lol).
With enough warning time you can prepare for pretty much every single weather disaster. That is the most important reason for forecasting.
That all being said, people like to prepare for weather that isn't going to kill them too. It is nice to have an umbrella when you need it, and farmers like to know what days is the best to plant, etc. etc.