What are the primary goals of creating a building model? Who are the key stakeholders?
The key stakeholders are the client, contractor, and architect.
The client needs a design that they feel meets all of the specifications of the project. The model is a way for the architect to show what factors they have considered in making it. The client cares most that the model describes a beautiful, livable building.
The contractor needs a design that is easily understandable and buildable given their level of experience. If the design is clear, the contractor will run into fewer problems in the future. They care most about precision and robustness.
The architect needs the building model to save their progress and make changes on the fly when needed. Additionally, the saved model can be a template for future projects. They care most about usability.
Many door and window manufacturers provide Revit families for doors and windows that you can specify for your building design.
Manufacturers benefit because architects in the design phase who have not yet chosen building components will likely select the most easily available components at the time. By making CAD of their components accessible, the manufacturers increase the likelihood of an architect picking their products.
Designers benefit from manufacturer-provided Revit families because it makes the design process much easier. Rather than having to do their own research to know the dimensions of products they're unfamiliar with, they can simply download those products online and understand how they work immediately. It speeds up the design process and decreases the risk of making a mistake.
What are the advantages of getting the Revit component families (for furniture, equipment, and fittings) directly from the manufacturer versus from an online sharing website like RevitCity.com?
The good thing about getting components directly from the manufacturer is that they're more likely to have the most up-to-date versions of all of their products. If you're going off of a third party, the components might not be sized correctly, or they may no longer be sold.