The active mastication improves the state of the hippocampus and its neurons. One study showed a correlation where people with fewer remaining teeth had lower levels of cognitive function and a higher risk for acquiring dementia. The reason why this correlation exists is because our brain derives a lot of beneficial stimulation from the oral region of our body and so chewing is one action that provides that stimulation.
Some studies recently revealed that mastication leads to increases in cortical blood flow. link