The application of salt and nitrite is actually our first line of defense against the growth of spoilage bacteria as in many cases (home production) there is very little we can do about a selected meat's bacteria count except making sure that the meat is fresh. As the sausage slowly dries out it loses moisture but not the original amount of salt which remains inside. As a result, in time the sausage becomes much saltier to bacteria. In about 3-6 days the Aw drops to about 0.95 and the sausage is microbiologically more stable as some pathogenic bacteria (for example Salmonella) stop multiplying now.
Using a combination of different hurdles is more effective that relaying on one method only. For example the first hurdle is an application of salt and sodium nitrite which eliminates some of the microbiological spoilage. This will not be enough to produce a stable sausage if we don't follow up with addidional hurdles such as lowering pH (increasing acidity) and then lowering water activity Aw (eliminating moisture by drying).