The reason why you would want to get Premium LinkedIn or Sales Navigator is that there are no commercial view limits as with the standard LinkedIn. The commercial use limit is based on your search activity since the start of the current calendar month. It takes into account non-name searches, mobile searches, searches outside your network, frequency of searching, viewing suggested profiles, etc. This limit resets on the 1st of each calendar month.
Also, Sales Navigator offers more search filters which let you filter the results by the postcode or the company size, which means that you get more relevant leads to contact.