What is pathetic fallacy and why is it used ?

Pathetic fallacy is a specific kind of personification - that is, attributing human traits to abstract concepts or non-human entities  - which relates to the connection between a human's internal emotions and their external circumstances. Often, this takes the form of a character's feelings being reflected in their surroundings, such as the weather. This has a long history of being used in storytelling. For example, in the Anglo-Saxon poem The Wanderer, the main character is exiled and lonely, and the rough, cold seas he sails in reflects that. 

It's common in the work of the Romantic poets (like Keats and Wordsworth) but there are a lot of good examples of it across literary forms and periods. For example, in Jane Eyre, Rochester proposes to Jane under a chestnut tree - the next day, there is a terrible storm and a lightning bolt splinters the tree in half. This reflects the uncertain state of their union, and foreshadows their later troubles, too.