Ivan's study group is preparing for a history test on world war ii. his great-grandfather was a journalist during the war, so he shares some of his great-grandfather's war stories with the group. how does this most benefit others in the group?

I believe the answer is: by offering a new perspective and more information

New perspective and information would generally ignite an opinion or an argument from members of the study group.
This would make them to explore various ideas within that topic and together they would deepen their overall context on that topic.

Afine-grained igneous rock forms as a result of slow cooling.