[zaarn]

When you build a small house, you don't account for the curvature for earth. Same for when you walk down the street.

When you build a runway for a plane or a long bridge, you do.

A model is not necessarily useful in all contexts. People still use the flat earth model in useful ways because it's simpler to assume the earth is flat in some situations. Of course, once you go beyond the capabilities of the flat earth model your numbers will wildly diverge into the realm of useless while the round or spherical models provide useful numbers for longer.