This is an example of a direct network externality. In these cases, the value of an operating system is higher when there is a large number of applications that can run under it. The combination of these effects makes the value that a consumer receives from an OS higher when the sales of the OS are higher. Similar reasoning shows the existence of network externalities in any good that requires a distribution or a repair network, or, more generally, requires a complementary good. It seems that many industries exhibit network externalities.
Some products have no value if there is no network.
For example, a fax machine is useless unless there exists another fax machine to communicate with. In contrast, a computer program, say WordPerfect, has value even if no one else bought it. Network industries and network goods are a very significant part of the US and world economy since they encompass key industries, including telecommunications, computers, electricity, and transportation. These industries, especially computers and telecommunications are growing very fast. They are driven by the steep reductions in the costs of integrated circuits, the progressive digitization of many functions, and the wider use of programmable multi-purpose devices.