On a recent engagement I prototyped a scalable content generation backend. Technically, it is an extension of the Rails build-a-blog project that everyone does in a tutorial, and it is trivially within the reach of every other Rails developer with 4 years of experience. The value I add above and beyond being able to implement a scalable content generation backend is a) knowing why you'd want to do it in the first place, b) creativity in applying it to whatever business is under discussion, c) being able to describe in detail what process one would need to use to use it in production, and d) being able to convince the relevant people that a + b + c = money.

Consulting engagements also have a number of risks associated with them. Providing the perception of diminished risk is worth value to the client. Many developers could implement A/B testing, for example. (And they should, because it potentially prints money for the right clients.) There are a variety of ways a hypothetical developer could cause a client to believe that an A/B testing engagement with them is less risky than it could be. One is to have written a widely used A/B testing framework.