What does a social media agency do?
A social media agency manages brands' social accounts on their behalf: it sets content strategy, produces posts and ads, publishes, manages the community and reports results. The agency's value is turning what a brand does in scattered pieces on its own into a single, consistent, measurable operation.
The hard part is scale: producing content for one client is easy; producing, scheduling, publishing and reporting for ten clients every week is a separate operation. What grows an agency is being able to multiply that operation without it falling apart.