Any SE should address some specific subdomain, and in this case, I believe we address the desires of people who want an evidence-based approach to being healthier. Ideally, we translate academically rigorously accepted information into digestable formats for people without the knowledge, skill, or background to understand and interpret the context of research papers.
This isn't really a place for academics to talk about new procedures, for people to get diagnoses, or for getting a quote on health insurance, but we address a very specific need: to be healthier- whether for yourself or for others (first aid / vaccination / etc.)
The problem is that health.SE wants to be at a level of academic rigor that goes beyond Quora and yahoo answers, but is still vulnerable to bro-science quality Q&A's.
Ideally, I believe we want to focus on bridging the knowledge gap between people who are studying health academically and people who want to live healthier, not so much healthcare professionals (who, though we want to help, we really don't have many means of maintaining quality posts for them), such that anyone can live a healthier life, and find a place with credible responses of academic rigor.
We kind of learned the hard way these past few years that we just won't have to resources to support high-level academics consistently- not enough of us are real professionals, but many of us know how to research, read papers, and understand the implications of scientifics studies (and where those limitations lie).
In this age of fake/questionable/biased information and popular semi-logical but wrong science, we're trying to separate the wheat from the chaff for people. I think that health.SE evolved to try and bridge this gap, for those who don't have the time/energy/background to evaluate research and back up our answers with strong support.
:) Making information not just accessible, but comprehensible and evidence-based.