In a professional production environment, a large number of tools can be used in projects. Source material can come from many different programs and in a variety of formats or it might be easier to create content in one program over another. For this reason it is important to understand how Blender can be used in concert with other programs, closed or open source.
Ari HÃ¤yrinen pointed us to a tutorial which uses Inkscape to construct a floor plan from a hand drawing and then model the building based on the floor plan in Blender. You should check out Ari's two virtual reconstructions found here. And even though they weren't modeled in Blender, it demonstrates the use of 3D for the purpose of architectural history.