The reasons vary, depending on who you are. Maybe you'll see a good reason for yourself among the following:
The feels-good reason: It simply feels good to be outside snooping around, moving here and there, keeping your eyes and ears open, looking for new things while trying to understand old things, as the sun shines, the wind blows, birds sing, butterflies flit, mushrooms emerge, bugs scamper...
The brain-food reason: Any scientist or serious student can tell you that once you begin studying nature it's really true that "the more you know, the more you realize how little you know." Also, the more you know about nature, the more exciting it gets to learn more. Anyone who really gets into nature will never be bored.
The it's-beautiful reason: It's a blast to see the different ways sunlight can filter through a bright leaf in fall, to hear the music of insects deep in a summer's night, to discover the intricate patterns of venation on the wings of tiny insects, to thrust the mind deep into the throat of a weed flower...
The get-a-philosophy reason: Once you realize that nature is so complex, huge and beautiful, you can't avoid wondering what it all means. Why are we humans placed in the midst of all this, and what are we supposed to do about it? As the ancient Chinese showed us, nature was suggesting answers to those questions long before the world's formal religions came into being.
The understanding-the-Creator's-plan reason: Some people believe that the Creator's plan for the Universe (including the plan for us humans) can best be determined by studying that tiny corner of nature present on our little planet, the Earth. Are not the lessons of recycling, of preserving clean air and water, of sharing the ecosystem with other living things, of the individual's responsibility to the broader community, etc. at least as important as the teachings of regular religions?
The doing-what's-right reason: If you believe that nature teaches us important lessons and if you are concerned about the plight of Life on Earth, then you would do well to study nature closely. For, it's clear that if present trends continue (the spoiling of clean water, clean air and open space, the poisoning of the ecosystem, the extinction of species) disaster for us all will result. What is to be done to turn things around? The Earth's young people must come up with that answer, and surely there's no better place to look for answers than from the study of nature.
The survivalism reason: If you believe that for some reason the infrastructure supplying you with food may eventually break down, it makes sense to develop enough familiarity with the basic principles of nature to be able to grow, prepare and store your own food. To do this effectively you need a general idea of what soil is and how it functions, how to germinate seeds and protect plants from an enormous range of insects, diseases and nutrient deficiencies. We have a special page on backyard gardening to get you started.