Susana isn't super active in this novel—her big contribution to the plot is dying after acting crazy for a long time. But that doesn't take away from her importance.
She is Pedro Páramo's obsession, and her death is what causes him to finally condemn Comala to death. He spends his time talking to her, even though she's absent, saying things like, "Hundreds of meters above the clouds, far, far above everything, you are hiding, Susana" (7.8). That would be sweet if it weren't for the whole I'm-sad-so-now-I'll-starve-Comala thing.
The character of Susana works kind of like a Helen of Troy figure. In Greek mythology, Helen was a woman whose beauty caused men to go nuts over her: Guys were so infatuated that they ending up starting the Trojan War. Susana, too, is inspiringly beautiful enough to prompt Pedro to murder.
She also fills the role of madwoman, another important role (actually, she shares it—there are more than a few nutcases in this town). As in many other works in the Western Canon, the madwoman speaks the truth. Just check out another Greek woman from the Trojan War named Cassandra. Cassandra was able to accurately predict the future, but was cursed by having no one believe a word she said.
Although everyone thought Susana was a NutterButter when she was alive and kicking, it's Dead Susana's voice that everyone is interested hearing in the graveyard… because they want to know what the real deal was with Pedro Páramo.