Imagine you want to build a City a. You come to a green empty field. Nothing is done, everything is possible. b. You come to a town. A lot has been built, a l

Imagine you want to build a City a. You come to a green empty field. Nothing is done, everything is possible. b. You come to a town. A lot has been built, a lot of space used, but there are still many fields around the town to expand to, and build new things. c. You come to a City. All the space is used. Everything you build must work with has already been built. Some things can be built, but only if it does not inconvenience what is already there. Software as a whole is approaching/already in stage C. Sure you could make something that does not connect with anything that exists. But what use is that to anybody. People want to connect with the thing they already use. This metaphor works with a lot of things. Operating Systems, the Cloud, even things like Game Libraries, ala Steam. To produce value, you need to work with what's already there.