Theory:

The importance of forests can be summarized below.
 
Forests provides various useful products:
  • Forest products are the things that are obtained from the forest; these are very useful to humankind.
  • Products obtained from forests include wood, gum, honey,  sealing wax, catechu (extract from acacia trees that are used for medicinal purposes), oils, fruits, medicinal plants, natural rubber, spices, cork, fodder for cattle, etc. 
  • Wood is used for fuel as a source for cooking. It is also used for making items like furniture, matchsticks, paper, plywood, boxes, and many other things.
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Forests maintains the balance between oxygen and carbon dioxide:
  • Plants take in carbon dioxide (CO2) and release oxygen (O2) in the process of photosynthesis. O2 released by plant utilized by animals for breathing.
  • The photosynthetic process thus maintains the ratio of O2 and CO2 in air. Thus, forests are termed "Green lungs".
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Forests maintain the water cycle :
  • Plants suck water from the soil through roots and release water vapour in the process of transpiration or evaporation from leaves.
  • Water vapor helps in the formation of clouds. It is a forest that brings in rainfall on the Earth.
  • Forests maintain the water cycle and fulfills our freshwater requirements.
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Reference:
https://in.pinterest.com/pin/521713938058584228/?nic_v2=1a6P4vmLe