It's important because the toposurface affects the way we create walls, and might determine whether or not we need to use a building pad. We don't want a situation where we have walls below grade and we don't know because we haven't made a toposurface.

From a constructability point of view, it makes most sense to adapt the building to the terrain - it can be very expensive to change the topography of a site just to build on it.

The major reason is to comply with code - it's also a lot easier to climb up stairs with certain dimensions. Stair design really affects the accessibility of buildings and playing around with it creates unequal access, and can also be physically exerting on the building's users.