To tell the truth about Kurama's attack on Konoha 16 years ago, since what happened there would eventually lead to Itachi being forced to kill the Uchiha. Obito lied to Sasuke about it and Orochimaru might see him as a threat, so adding Sasuke and the Hokages into the fight against him would help eliminate Obito as a threat, and by relation, prevent Madara from becoming completely revived, again clearing away a threat. Also maybe to ask Sasuke about Naruto and get him to see reason through it.
Though I wouldn't be surprised if somehow if Sasuke does get angry about Obito's betrayal that his anger awakens Perfect Susano'o.