I'm a contributor to OpenStreetMap, and last year I helped Code for America import thousands of buildings.

OSM is rightfully skeptical of uploads. Even when cities give specific permission to OSM, it's up to the community whether a large import is helpful or risks drowning out local mappers. If anything, this site would be helpful to download a subset of the city's buildings as KML and use JOSM to upload.

This site exists as a custom mapmaker. It's easier for a non-GIS user to get data out and make custom maps compared to OSM APIs and tiles.