Why is it important to accurately model the land features of your project site with a toposurface?
It is important because it influences what parts of a building will be underground. If you design a basic two story building with windows or thin/weak material everywhere and then place it on a hillside site, most of the first floor will be underground. The windows on the backside would be useless and the materials would be too weak to hold up underground.
When designing a project...
you should usually adapt the building to the terrain, but there are cases when it is necessary to adapt the terrain to the building. In most cases I actually think it takes a bit of both. Given a reasonable terrain, you should try to design around slopes and topography changes, however there are also times when the terrain is not reasonable or your building requires certain things that the given site does not allow. A healthy balance of respect to the site and respect to your work is key.
Why do stairs follow specific proportions with a set relationship between the tread length and riser height?
The first reason is that the specific proportions are safe, common, and follow building codes. The second big reason is due to the human gait and consistancy. People are used to comfortable and consistent stairs, so the specific proportions limit trips and falls on the stairs.
Building modeling can help prevent mistakes by visually displaying how the stairs will meet the floors, how many steps are required, their depth and height, and the overall space the stairs require to function