Needed is a strong word, but what this provides are guarantees when reading the code. Much like you know that control flow will come back to the function you're reading after calling another function. We didn't need to know that control flow will resume in the calling function, but it turned out to be very useful.

In some existing languages, yes. But it’s easy to imagine future languages (including future versions of current languages) that disallow creating nurseries outside of a function scope.