Some projects can get started as a lower-cost alternative to proprietary systems. In this case, the developer company does not have (at least in the beginning) any plan to get external income related to the sale of the software or services related to it. The company develops some system because it is useful for them, and later decides to make it open source, and distribute it widely, just to benefit from the open source development source. Probably they will get some contributions, improvements and bug fixes from external developers interested in the software, and some bug reports. Later on, the product may even reach some market acceptance, and the developer company could even get some economic benefits from it.
For instance, a large enterprise with several thousand desktop computers can decide to create some software internally, and to make this software available under an open source licence to get the benefits of a larger base of developers that may be interested in helping out. The Cisco printing system is a good case example of this kind of model.