הַפְטָרַת שַׁבָּת הַגָּדוֹל תשפ"ו - Why is Elijah the one sent before "The Great and Terrible Day"?

From Jewish History with Rabbi Dr. Dovid Katz by R Dr. Dovid Katz.

Will תְּחִיַּת הַמֵּתִים occur for the benefit of the Dead, or the benefit of the Living?