Quite often when you are working in Revit, you are modelling based upon existing information. Be that a topographical survey or measured survey of an existing building, etc. The typical workflow is to bring in the existing information into your Revit Project and use that as a template to create your model.
A typical workflow would be to bring in a DWG file and use the Pick Lines tool in order to quickly create your model. However, we can also now snap to elements in a vector PDF file that we have imported. In the above video I show you how to do this.