There are several reasons why someone might want to kill weeds:

1. Aesthetics: Weeds can be unsightly and detract from the appearance of a lawn, garden, or landscape. Removing weeds can help maintain a tidy and visually appealing outdoor space.
2. Competition for resources: Weeds compete with desirable plants for resources such as water, nutrients, and sunlight. When weeds are left unchecked, they can reduce the growth and productivity of crops, flowers, and other plants.
3. Safety: Some weeds can be hazardous to humans and animals. For example, poison ivy can cause a painful rash, and some thorny weeds can pose a physical danger.
4. Invasiveness: Invasive weeds can take over an area and displace native plants, which can have negative ecological consequences. In some cases, invasive weeds can even threaten entire ecosystems.
5. Health concerns: Weeds such as ragweed can also cause allergies and other health issues, so removing them can be beneficial for people who are sensitive to them.

Overall, while not all weeds need to be removed, controlling them can help maintain the health and beauty of your outdoor space, protect your plants, and reduce potential hazards.