You get a smaller size, which is not only nice for transporting purposes but also for shrinking the attack surface. The same reason container images leave out a lot of the base OS. Why bring in more than you need?
The “multiple releases” section is referring to building a release with varying components. You might have a project with apps only necessary in particular environments. In order to have those included and started you build a release that contains them (maybe you have separate database apps or separate release for a server and a client).
Which also brings to the issue of consistent startup process. You don’t necessarily have a single application you can start that will start all other applications required. Now you must write your own startup code that ensures all top level applications start.
Why do this and include libraries you don’t need when you can just build a release? 
Plus it means you aren’t tied to a container.
