A more direct answer to your question is: testing is really the only thing you are not permitted to do without a Doctorate, as well as some legal assessments. But, a PhD will allow you to do things at a higher level. So, with your Masters you can teach undergrads, with your doctorate you can teach grad students students. With your Masters you can testify/do some court evaluations, but with your Doctorate you will get the more serious cases and can do all types of evals (except med evals). With your Masters you can supervise people and maybe be a director of a clinic/dept, maybe. With your Doctorate you have a much better shot at being a Director and can supervise more people. With your Masters you can administer some testing/assessments, with your Doctorate you can administer and interpret more in depth testing and evaluations. With your Masters you can work in the ER but report to a PhD or MD, with your PhD you can be the supervisor, or co-supervise. With a Masters you can work in a research lab, with a Doctorate you can run a research lab.
On an aside: monitoring the cognitive functioning of a pt during a craniotomy sounds fascinating. What an awesome experience. I never realized that's something psychologists did. Very, very cool.