Many applications provide a front end using a GUI framework that comes with the compiler or a third party. Some software uses special-purpose libraries to do things like statistical analysis or mapping projections. There is less risk in using a third-party component if it is open source. This is particularly true of components developed by small companies and individual developers. It's very difficult to develop software components and make money renting them.
Developers can see what an open-source component does and how it does it.
If a component is open source, its owner cannot rent it. However, money can be made in support services, and the people best poised to do so are developers. A tiny outfit without the credibility to rent a component may find that the best way to make money is to make it open source. It might be easier to attract contributors to an open-source project for a component than to a project for a narrow application. The users of a component are programmers, and they make up a natural pool of potential contributors whereas the users of a narrow application might be, for example, dentists.