"Mr. Trump felt it was important to do, so he did it," Paul Manafort told CNN's Chris Cuomo on "New Day."
Trump phoned into the network Monday night, interrupting the network's coverage of the first convention night. Two survivors of the Benghazi attack -- Mark Geist and John Tiegen -- were recounting their experience while Fox News aired Trump badmouthing Ohio Gov. John Kasich for his decision not to attend the convention.
The Benghazi attacks were a key theme of the convention's opening night, which focused on national security.
And although Manafort said he did not know that Trump was planning to do an interview, the candidate decided to go forward after being "moved" by the speech of Patricia Smith, mother of Benghazi victim Sean Smith.
"I mean he was moved by her speech, too. Last night was a very emotional night," Manafort said.
"How was he moved by it when he was on T.V. at the same time she was giving it," Cuomo asked.
"He had the TV on and he was watching it," Manafort responded.
Trump discussed the Kasich situation because he wanted to take his message beyond the convention, Manafort said.
"We were not just preaching to the converted last night. We were talking to America, American voters throughout the country," he said. "Many of whom were listening in to understand many of the themes we're going to be talking about during this election. They heard a very clear message."