According to DAL, the Bombardier C was to be replacing larger than 50 seat RJs, so those Larger RJs could be replacing 50 seat RJs, thus upgauging the fleet.
It does, however, have a VERY long range, so it could do many flights that were impossible on any RJ (or MD-88/MD-90/B-717), and even some small Airbuses and the Boeing 737, I believe. An example that Bombardier used, was a typical load nonstop from London City airport to Montreal using the smaller CS-100