A content marketing agency can help your business grow through successful execution of a content marketing strategy. The agency could provide any number of services, to complement or replace internal resources.
This could vary from strategy, creation, optimization, analytics, reporting. Or all of the above. It can be a big advantage to access higher calibre talent, or specialists in an area, without having to take the full time cost on internally.
Each business should strive for a balance between the internal and external resource. So be open minded over time as to which capabilities need to be in-house as compared to externally.