The true strength is in the open interfaces and common protocols that enable competition and choice, followed by the free-to-use libraries that establish a foundation upon which we can build and iterate. This helps us to stay in control of our hardware, our data, and our destiny.
Practically speaking, there is often more value in releasing something as free software than there is to commercialising it or otherwise tightly controlling the source code... and for these smaller tools and libraries it is especially the case.
Many bigger projects (eg. linux kernel, firefox, kubernetes, apache*) help set the direction of entire industries, building new opportunities as they go, thanks to the standardization that comes from their popularity.
It's also a reason why many companies release software as open source too, especially in the early days, establishing themselves as THE leader...for a while at least (eg. Docker Inc, Hashicorp).