There are three kinds of external help a marketing department can get.
There is the help that is valuable because it provides a competence that is missing. This is typical when you hire a freelancer, for example. Perhaps you have a small team, you lack some skills, you want something specific done that you cannot do yourself.
Then, there is the help that is valuable because it gets things done. Most agencies fall into this bucket. They do not really deliver mind-blowing results, they might or might not have specific competences, some might actually argue they could have gotten pretty much the same outcome by doing the job internally. But the truth is, the team simply does not have enough bandwidth, or it is not well organised, or its skills are not well mapped.
Finally, there is the help that is valuable because it delivers quality. It might be a freelancer, it might be an agency. But in this case, they are at the edge of their field, they are doing things that not many others know, they are reinventing a particular tactic or the way it is approached. Things might get difficult, because the counterpart is somebody with convictions, ideas, opinions, and they might not be willing to simply do what pleases the head of marketing. They repay these difficulties with an outstanding job.
Now, the fact is that often we approach external help in marketing with unrealistic expectations. And so, when we hire for competence, we do not want to do the project management work that is necessary. When we hire for project management, we are unprepared in feeding the right information at the right time because we do not have them. When we hire for quality, we are not ready to change our assumptions and beliefs, and potentially redefine strategy and tactics altogether.
Who are you hiring to help you out? Are you aware of what that means?