Updating the screen with the latest values of chromosomes (variables) and fitness values can be a time consuming process when done repeatedly.  If you don't care to watch the progress of evolution, then setting the screen update to Never is the best choice.  The Always setting updates the screen after every generation, refreshing the variables and best fitness results.  A compromise, which is the default, is Smart updating.  Smart updating updates the screen only when the latest fitness value is better than the one on the last screen update.