In some areas/regions of the game, there are a lot fewer players around. So to encourage people to visit less populated regions, why not give bonus SP and rewards to people who do a GL in that area? For example, if half the population does GLs in the desert town, the coastal and forest towns get the bonus.

Obviously things like rank need to be accounted for, but in general, if there are rewards for spreading out, players will be more likely to do so.