Consultants bring perspective
At a recent speaking engagement, I was asked what I think is the greatest value a consultant can bring to a project (in this case, choosing a new association management system). I think the greatest value a consultant with significant experience can bring you is perspective.
For example, when I work with clients on choosing a new AMS, part of what I'm looking for is what make the client "weird." That is, what makes this association unlike most other associations. Because that is likely where the vendors are going to have the toughest time providing solid, out-of-the-box technology. And the reality is, unless you've worked with dozens or hundreds of associations (like I have), you and your staff are unlikely to know what makes you weird. Because it won't seem weird to you; it's just what you do every day.
So if you're wondering what value a consultant might bring, perspective is something that you're unlikely to be able to provide from the inside.