Broadly speaking, an IT engineer who works with free software has a similar profession to a chef, car mechanic, plumber or lawyer.
Law firms work with a knowledge and understanding of legislation that is as free and widely available as free software could be. Clearly, their business model consists of raising revenue from a complementary product, which is their expertise or in-depth knowledge of the law, their ability to adequately organise the information set down in legislation to defend their client's interests, which are things that their clients cannot necessarily do.
Ultimately, the lawyer incorporates the right ideas into the right product for its client (defence of the latter's interests).
Similarly, computer engineers who work with free software offer clients their expertise, the ability to meet their need to organise information in a specific way and process data by harnessing the intrinsic possibilities of the free software available, or, if necessary, by developing additional code.
Thus, we can see how a given economic sector (legal services) can even have different levels of information (corporate, law and Aranzadi on one level and law firms on another), which gives rise to multiple business models that simultaneously coexist.