The more players are on the server the more it causes weight to server. By reducing the max player count it gives players better performance and there is less lag to people with slower computers
Essentially what @Drama Lama said. It allows the server to have to process less data and dish it out to fewer parties, therefore meaning it has a lighter load.
Only temporary, I hope, whilst the mass kick issues are ongoing.
Bohemia are aware and I believe are trying to solve the issue in ho