The "War of Reckoning", which started out as a way to to avenge King Gavilars assassination, quickly turned into a political competition for gemhearts once the Highprinces discovered the amount of chasmfiends in the Plains. These gemhearts are essential to the economy because over time, the gems crack and need to be replaced. Before the Alethi knew that the chasmfiends had new gemhearts how did the circulate new gems into the economy as they would all eventually crack and be useless?
Gemhearts exist in many of the native animals of Roshar, not just chasmfiends. In fact, as stated in https://coppermind.net/wiki/Gemheart
"Gumfrems, small chull-like animals, are raised by some farmers in Alethkar for their small heliodor gemhearts that can be harvested and used to Soulcast meat."
In other words, the Alethi would both hunt and farm other animals for their much smaller, much less valuable gemhearts.
Chasmfiends were discovered to have enormous, extremely valuable gemhearts, and so they became a new, particularly desirable source of them.