The most common reason I make create CRC's follows this scenario:
1. I create a new part that has only the default datums and I assemble that in my assembly using the datums.
2. I create the geometry of the new part in the context of the assembly and along the way I create external references to the assembly, maybe from a use edge in the sketcher or a dimension to geometry that exists in the assembly and not in the part.
3. After completing the part, for whatever reason, I redefine the placement of the part and use the surfaces of the geometry as the placement features.
I have created a CRC. The part geometry was built with reference to the assembly AND the geometry of the part is now locating the part in the assembly. Which came first, the chicken or the egg.
In my world, I usually will break most, if not all external references in my parts when I have my design ready production. Usually at that point, I don't have any more CRC's.