Ah, I think the confusion here is because of the different meanings of the word "set".
If you don't select the deployables in the release template, it will have the default deployables available in executions. That is, if you do for deployable Foo and Foo is a deployable in the application, it will execute the code inside the block.
However, you can select specific deployables for a release and the for deployable Foo block will only be run if Foo is selected.