A Project is the file that Cinema 4D uses to create assets, render scenes and animations, create animation, and so on. The project and associated settings are also where you determine the various conditions that exist for your project.
Environment and User Interface settings pertaining to project length, drawing levels, project scale and units, UI color schemes, image input color profile and more can be potentially different for each project (assuming you want or need them to be). File paths for external assets like texture maps and rendering directories are also stored in the project.
You can also store authoring and copyright information in each project. This can be especially handy if you are working on a team, as opposed to a one-person show.