An agency that does social media for itself gets to learn and experience the finer points and nuances of the various platforms. Every platform has a different audience and they respond differently to different messages. A Pinterest member is not the same as an Instagram subscriber though they both love life in pictures. A LinkedIn member is quite different than a Google + member. Yes they have commonalities but you need to understand the differences to tailor your messages accordingly.
Outsourcing companies / digital agencies that do their own social media campaigns have experienced first hand the frustrations of sometimes their best ideas generating nothing in return, how to recover, how to adjust, fine tune and re-launch. If the agency doesn’t do their own social media promotion, it would be like going to a doctor who has read all the books but has never practiced. I’d rather go and pray.