In composite resin, decreasing the size of the filler particles serves what primary purpose?

Smaller filler particles (as in microfilled and nanofilled composites) allow for a much smoother final surface polish and luster, which is the main reason they are favored for esthetic anterior restorations — though this often comes with some trade-off in mechanical strength/wear resistance compared to larger-particle hybrid composites used in load-bearing posterior restorations.