Agencies should approach content marketing in line with their new business development strategy and plan.
It is one element of the promotional mix. Some choose not to blog or use social.
BUT as a veteran advisor of agencies on biz dev I can tell you one thing: Many suffer from the Cobblers Shoes Syndrome.

They can do marketing for clients but they fail to market themselves successfully.
They run out of time or budget or client work takes priority.

I run an agency and have found this to apply to us. Our solution is to have a "client" who is ourselves and then to treat them in the same way with team meetings, updates and timesheet billing.

Can you tell me more about why you're asking - are you an agency new business professional?