matrix-synapse-rest-auth is commonly used together with mxisd. The
uninformed might either

* not know that `matrix_synapse_ext_password_provider_rest_auth_endpoint`
  has to be set or
* set it wrongly to `http://localhost:8090` which doesn't work because
  matrix-synapse doesn't run on the localhost, but inside a docker
  container.

The correct value for that common use case is `http://matrix-mxisd:8090`
and is a better default than the empty string.