We all see what's happening on the battlefield. Everybody is puttin in WORK. But Edo Madara has unlimited chakra and he can't die. Naruto, Kakshi, Guy, Killer B have no sealing techniques. And if Obito turns good again, he has no sealing techniques either.
What if Sasuke approaches the battlefield and becomes the one to defeat Madara. Sasuke needs to do something big to contribute in this war.
Sasuke wants to avenge his clan and when he talks to Tobi, Tobi refers to himself as "Uchiha Madara".
Sasuke didn't know that Tobi(refering to himself as Madara at the time), helped Itachi with the massacre. Itachi did it because he was told to by the Konoha elders. But Madara did it because, "He had a personal grudge on the clan for selfish reasons". Keep in mind that Sasuke still doesn't know that Tobi is Obito.
"But Sasuke has no sealing tech either." Sasuke could have the Sword of Totsuka from Itachi because Tobi said that Itachi transferred all his eye techniques to Sasuke. That could be able to seal Edo Madara.
In Conclusion: Sasuke still thinks that Madara had a big part of the massacre and wants to get revenge. He also knows that Itachi wanted Sasuke to stay far away from the masked man(Who Sasuke thinks is Madara). This is just a theory though. I tried to be as logical as possible.