It is important to protect the environment because man-made disruptions to ecosystems can cause extinction, because pollution creates dangers for both animals and people, and because mankind owes the natural world a moral obligation. Many of the dangers to the environment come from practices designed to make human life easier but actually threaten the long-term health and prosperity of humans.
One of the most important reasons to protect the environment is because it helps protect different ecosystems. Over the years, humans have discovered that many ecosystems are more complex than was originally thought and are deeply connected to other ecosystems. Human changes to one ecosystem, whether through environmental pollution or deliberate manipulation, can actually put multiple species in danger of extinction, necessitating the need to protect the environment.
Protecting the environment also protects humanity. A great deal of pollution, especially ocean pollution, ends up affecting creatures that humans later consume, meaning that humans are ingesting toxins.
The final reason to protect the environment is that humanity has a moral obligation to the world and its creatures. Humans are responsible for taking care of the world, and protecting the environment is one way to be responsible stewards of the world entrusted to their care.