Manafort was brought into the campaign to get the needed delegates at the convention. For that, he was very successful.
Personally I think Manafort's role in securing the delegates was overrated. I remember there being much ado at the time about Cruz "stealing" Trump delegates in various states - especially the caucus states - but when Trump decisively won the Indiana primary on May 3 - it was all over for Cruz. That's why he dropped out that night and Reince Preibus (then RNC Chair) declared Trump the presumed nominee.
At the time, Manafort did seem like the "steady hand" that Trump needed but as many remember, he was unceremoniously dumped in mid-August in favor of the duo of Kelly Ann Conway and Steve Bannon. A move that was derided by most at the time as a "desperate move by a dying campaign" but seems genius in retrospect.
Something happened behind the scenes to cause Trump to abruptly cut all ties with Manafort. Perhaps soon, we will have the answers.