Why might the theme of movement be important to a historian?

IF I understand your question correctly, Historians find movement important because
-geography plays an important role. If people(s) move around, it could mean that their gathers and hunters, but if they stay and they build settlements, they might farm.
Geography can also play into what people believes in, which is why movement of a people can influence religion.
-Settlements can create type(s) of government that vary. 
-movement of a people can also tell architectural advances. For instance, Native Americans didn't use wheels or horses that Europeans did.