The Cubs have had a lot of good closers, such as Smith and Sutter, but seldom had much beyond that. There really wasn't such a thing as a set up man before the last decade or two, but the closest thing to that was the role filled by pitchers such as Abernathy and a few others. Beyond that, the pen was manned by guys that were not good enough to start, and when someone did perform well in relief, they were soon moved into the rotation.
Even more recently, relievers in the minors were usually failed starters, or starters that have had injury problems and weren't trusted with a long workload. But the most recent trend has been drafting pitchers with the intention of keeping them in the pen throughout their careers. Carroway is one of the first for the Cubs. I hope there are many that follow.