Google Maps and OpenstreetMap and many other web mapping products utilize the concept of zoom-levels to change the size of the map in a web client. Most people are however more familiar with the concept of map scales(i.e 1:1000, 1:50000 etc) when zooming in or out of a map or considering measurements. When using zoom-levels the scales are not very intuitive (http://api.geoext.org/1.1/examples/zoom-chooser.html). Even desktop GIS software maintains the concept of scales when zooming in or out of digital maps. So why is the concept of map scale not used instead of Zoom-levels in web mapping clients such as OSM or Google Maps? This would enable the maps to be much more functional for administrative purposes for example in land use planing where the use of set scales like 1:10,000 are required when printing to paper.
The biggest reason is that the scale is not uniform. Most maps are in webmercator, and it does not preserve distances. At a given zoom level the scale will be different at the equator and at an higher latitude.
The second reason is that scale really doesn't have much meaning when you talk about webmaps.