[partialzero]

Ports.

Others will probably provide some other good reasons, but to me, Docker is multiplexing the global shared resource that is typically singular on a machine (at least a cloud/vm) - network ports. Of course you can easily stand up multiple services (of varying or same software platform) but you will need to carefully negotiate and manage and proxy ports. Docker provides an abstraction that does this for you, where services are hosted under unique internal host names which are easily addressable.