Older versions of ArcGIS Online had the capability of displaying an AutoCAD DWG file that was included in the underlying MXD for the online map. You can still do that in ArcMap, but the functionality to carry that to AGOL went away at some point. That was a quick way to depict a set of complex geometry, though with limited intelligence.
If your online map is tied to a dynamic CAD file for an ongoing project, you now have to constantly go through a conversion process and keep recreating new shapefiles to display the new content. That's in addition to republishing the new map service.