In chapter fifteen, "The Gathering of the Clouds," Bilbo and the dwarves wonder what has been happening and where is Smaug. They had kept watch all night long and in the morning, Bilbo notices that many birds have been flying past the mountain. The old thrush reappears and unsuccessfully tries to tell the dwarves something. Balin wishes aloud for a raven, because "there used to be great friendship between them and the people of Thror" (231). Before Smaug, a famous pair of ravens, Carc and his wife, lived above the guard chamber, often bringing the dwarves secret news.
Later, the old thrush returns to the dwarves with a raven, "a most decrepit old bird. He was getting blind, he could hardly fly, and the top of his head was bald" (231). This raven is Roac, son of Carc, and chief of the great ravens of the Mountain. Roac "remember[s] still the king that was of old" (231). Roac brings the dwarves glad tidings, that Smaug is dead, but he also warns Thorin that lake-men's town has been destroyed and they will be wishing for amends from the dwarves' treasure. Lastly, the old raven offers Thorin some sage advice--he should "not trust the Master of the Lake-men, but rather him that shot the dragon with his bow" (232). The raven believes that Thorin should make a deal with Bard and restore peace to the valley.
Thorin, of course, is not interested in parting with his treasure whatsoever. Needing some back up for the looming fight ahead, Thorin instructs Roac to send flyers to his cousin Dain in the Iron Hills for help.
Roac is an old Raven, he is son of Carc and cheif of all the ravens. He was important because he was son of Carc who Balin had known as a child so they trusted him when he told them about the death of Smaug.