Software is the ultimate product when it comes to economies of scale.

The first copy of the software is incredibly expensive to produce, requiring the investment of many thousands of hours of design, coding and testing.

The second copy (and all subsequent copies) cost nothing to produce.

Open source software takes advantage of these economies.  As soon as one person makes the investment in software, all others can benefit.  In the corporate for-profit world, this is a disadvantage.  In the public sector, this is a great advantage.