In the prologue of Guns, Germs and Steel, why does Diamond suggest that Yali's question has "overwhelming practical and political importance?"

The phrase that you mention is found on p. 16 of Guns, Germs, and Steel.  To understand why Diamond says this, you would need to read a little bit further.  Diamond argues that Yali’s question has “overwhelming importance” for us today because the different rates of human development that Yali asked about have shaped and continue to shape our world.

Yali wanted to know why Europeans and their descendants were so much more wealthy and powerful than non-white people like his own.  Diamond says that the disparity between the wealthy and powerful countries on the one hand and the poor and weak ones on the other has shaped our world. He notes, for example, that Africa has been colonized until relatively recently because Africans were among the world’s “have-nots.”  The legacy of this colonization continues to shape African lives today as well as the relations between African and Western nations.  He points out that many languages are dying out today because the people who speak them are not rich and powerful enough for people to care to learn their tongues.  In these ways, the factors that Yali is asking about have shaped our world today and continue to shape many of the issues that we experience.  Because of this, his question (or at least its subject matter) has “overwhelming practical and political importance.”