There were multiple ways to do it as long as you stayed inside one very narrow ecosystem; JARs from the JVM, Python's virtualenv, kind of PHP, I think Ruby had something? But containers gave you a single way to do it for any of those ecosystems. Docker lets you run a particular JVM with its JARs, and an exact version of the database behind that application, and the Ruby on Rails in front of it, and all these parts use the same format and commands.