One zero too many and you start writing really big checks to the staff. One zero too few and minimum wage watchdogs come to get you. So you code up your business rules, giving maximum and minimum salary ranges for each position. Now, let's say the application is running in offices across the country. And the rules keep changing.
Salaries go up, positions change, and the minimum wage changes.
Keeping all those applications up to date is going to be a nightmare. The client-side proxy doesn't do anything except talk to the server. The server object encodes the business rules. To make a change, you update the server. Once you develop a servlet, you need to run it in something.