Why is Google App Engine Making --set-default a default parameter? -

so, in past, when i've used google app engine, i've utilized wonderful version system create integration, staging, , production environment. when we're ready go live, set production environment default, , starts receiving traffic @ standard appspot url (myapp.appspot.com), if it's not default have manually go version (dev.myapp.appspot.com) testing. awesome system pre-prod deployment full test.

however, went app engine deployment today , got warning:

warning: soon, deployments set deployed version receive traffic default.  keep current behavior (where new deployments not receive traffic), use `--no-promote` flag or run following command:    $ gcloud config set app/promote_by_default false  adopt new behavior early, use `--promote` flag or run following command:    $ gcloud config set app/promote_by_default true  either passing 1 of new flags or setting 1 of these properties silence message. 

so, apparently, google make every deploy of version, automatically promote default , accessible @ standard appspot url.

my question is, what's benefit in this? can't use a/b testing (since there's no b in equation here). can't use pre-prod test, because takes on environment right away. why want version push default? missing something? using versions wrong?

i'm genuinely curious.