The purpose of a roof is to keep the elements out of your living area. The roof is sloped or pitched to remove the elements in the best way possible. Snow exerts a great amount of dead load onto the structure of the roof. The snow load for a particular area is calculated and the pitch of the roof is derived from that. It is done to keep your roof from collapsing if the weight becomes greater than what the structure is designed for. So a contractor can cut costs by building a steeper roof and using less dimensional wood.

I realize this is a simplistic overview of roofs, but to delve any deeper would take you into engineering and I am sure you receive enough schooling. Good luck in your work and if you choose the building profession ....build them the correct way....