Why do we need roofs on houses? Why can’t we just build it flat on the top?
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Regardless of the shape, it is still a roof, if the inclination is less than 10 degrees it is called a flat roof. You do not what a roof totally flat because you what rain to drain away, if it starts out flat you can get a low spot so you want some inclination to avoid standing water.
The amount of inflation you need will depend on the type of roof material. Roof tiles for example have a minimum inclination requirement to be watertight.
Another factor is cost. The roof needs to be strong enough with for example wood you need a lot less material if you make a triangle and then lost a flat beam. That is especially the case if it needs to be able to handle snow loads.
The result is that quite often a roof with a triangular crosssection is a cheap and efficient way to build a roof on a small building. This is especially the case with traditional building materials.
Quite flat roofs are very common in commercial buildings. Concrete and metal beams can be used. For a building with a very large footprint flat roof might be the only practical option because an inclined roof would be very high
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